In American football you have to have at least 7 players on the line of scrimmage. You can have more, but some of your players are then "covered" by others and are no longer eligible. What's the point of this rule? What if you allowed the offense to put a few people on the line of scrimmage as they liked? Or, if it's necessary to have five offensive linemen on the line of scrimmage for some reason, why force two receivers to be on the line as well?
The simplest answer is exactly as Joe said: because them's the rules.
Most rules relating to offensive formations are designed to help out the defense; it's generally understood that because the offense knows what they're going to do to try to move the ball and the defense doesn't, the defense is at a significant disadvantage in their job. To assist the defense and thus help balance the game to prevent every game being an offensive shootout, the offense must give certain cues as to their intentions based on how they position their side on the field.
More specifically, the development of the "forward pass" in American football, differentiating it from most other games in the family which require laterals or rearward passes only, gives the offense a huge advantage in unpredictability. To balance this, the definition of a legal forward pass itself (especially the fact that it must not bounce off the turf; theoretically a lateral or rearward shovel pass can be bounced) as well as new rules regarding formation, give the defense a few clues about what the offense is planning, or at least how to attempt to defend it.
The "seven down linemen" rule makes it easier for the defense to identify eligible receivers; players who may proceed beyond the line of scrimmage and then catch a pass from behind it. Essentially any player that takes a "three point stance" on the line of scrimmage is ineligible, and may neither be the first player to touch a forward pass nor may they proceed further than 3 yards downfield prior to the ball crossing the LOS.
Now, the "seven down linemen" don't have to be the positions you'd expect (namely center, left/right tackles and guards, then a wide receiver on each end). The "two tight ends" formation is a very flexible one that plays with the typical expectations of a defense, putting all seven down linemen in the center of the formation allowing either or both of the tight ends to either block or move downfield for a pass attempt (or both). It allows for more running and short passing options than the 4WR formation, and allows a team to grind its way down the field 5-7 yards at a time simply by keeping the quarterback protected and the defense guessing. Here's one variation:
The disadvantage is that this formation puts most of the offense right in front of the "core" of a typical 4-3 defensive formation (4 defensive linesmen, three linebackers, two corners, two safeties). The defense can drop into "cover 6", with the outside linebackers shadowing the tight ends, zone coverage by corners and safeties, and the middle linebacker watching for the run, or they can "zone blitz" bringing all seven core defenders to stuff a run and prevent a long pass opportunity while keeping the corners and safeties in coverage.
Although the other answers are right in that things are that way because of rules they don't really tell you why - or at least correctly.
uniform numbers - This is purely for the fans. Fans could easily tell which position a player plays. It is ludicrous that at any level that a defensive player needs to see a jersey number to tell who is eligible. I have coached pee-wee through college. My 9 year-olds will tell you the eligible receivers after two weeks of practice and there are no jersey number restrictions at that age. On the flip side of this you could just issue numbers 80-99 to all of your offensive lineman and receivers. I am surprised Belichick hasn't done this. It would be somewhat legal for game time but the NFL would fine him heavily.
7 men on the line of scrimmage. This is not an original rule. This rule actually varied across different leagues in the early 1900s. It was made "permanent" for player safety. Imagine two stacks of 4 players in front of a RB that is pitched the ball every snap - would be like a kickoff return every play. Even in peewee football 4 players in the backfield is dangerous when I set up a shotgun full house backfield and send three lead blockers at your poor nose tackle.
Why not let the end WRs/TEs line up where they want? Because coaches will use that grey area to help spring their extra backfield personal to crack block the end in the back and so on. (remember there is no clipping on the line) You let me put two extra guys in the backfield and that is a huge advantage.
The other part of this is just the essence of the game. The 5 blockers vs 5 WRs is balance. If there were less blockers the game may look like flag football, the field might not be wide enough, there would be more blowouts, more emphasis on QBs (meaning a team with a bad QB has no chance at all like they have much of a chance now) and so on. The reason why football is so well received is that there are positions for certain types of players. It is a chess match. This balance is well crafted in the rules, even if the rules just "got lucky". But I doubt the rules just got it lucky - I am sure there were some very bright people thinking about the game they were crafting.
In part, the answer is because it is in the rulebook. Often these rules arise over time in the early part of the sport simply because it was done that way, and then codified into rules. Sometimes rules even refer to the "Traditional" way of things directly.
In this case, that is probably part of the reason: football teams lined up that way in the 20s and 10s, and so it became part of the rules as they evolved. The forward pass wasn't originally part of the rules of American football (it was originaly more like rugby: running and lateral/backward passes only). Thus some rules - likely, including who is eligible to receive passes - came later, and came in an attempt to balance the game. Allow all 11 players to catch passes, and it's hard to defend, so the offense would have a great advantage; so you limit the offense to only having six players eligible to receive passes.
The specific reason for "eligible players" and "ineligible players", as well as the number who line up on the line of scrimmage, has to do mostly with deception (or, avoiding deception). See for example how the Patriots played in the AFC Championship last year; they deceived their opponents by changing who was eligible and who was not in odd ways, and that led to quite a bit of confusion on the defense. As such, the rules were changed this year to prohibit that.
So, the defense knows from the snap who is eligible to receive passes (and thus, who the individual coverage assignments are) in two ways:
- Unless declared, eligible receivers are 01-49 and 80-89 numbers, others are ineligible. This works both ways; normally-eligible must declare as ineligible just as normally-ineligible must declare as eligible.
- The defense knows that, of the players on the line of scrimmage, the only players eligible are the ones on the end of the line on either side. All players off the line of scrimmage, plus these two specific players, are eligible to receive passes.
If you allowed eligible players to line up wherever they wanted, and allowed ineligible players to not be on the line, it would make it more difficult for the defense to determine who they should cover.