In addition to the historical reasons, the reason that not only do you have to have 7 men on the line, but you have to have a particular seven men, is to avoid confusion about who is eligible to catch a pass. From the rulebook, rule 7 section 5:
Section 5 Position of Players at the Snap
Article 1: Offensive Team. The offensive team must be in compliance with the following at the snap:
(a) It must have seven or more players on its line (3-19-1); and
(b) Eligible receivers must be on both ends of the line, and all of the players on
the line between them must be ineligible receivers.
(c) No player may be out of bounds.
Thus, not only must seven men be on the line, but those seven must be the 5 ineligible receivers (the offensive line, who may not be the first to touch a pass, and must stay behind the line on passing plays prior to the pass being thrown) with one eligible receiver on each side of them. You'll hear fouls periodically saying "The receiver was covering up the tight end" and similar, which means a WR was on the line and a TE inside of that WR was off the line (typically because the TE went in motion and the two of them didn't properly reset).
That helps the defense, as they then know which five players they don't have to cover in pass defense. Otherwise it would sometimes be confusing as to which of the 6 linemen was eligible without looking at their numbers (which isn't always possible when they're in their stance).