In basketball, why don't players block three pointers by putting their hand above the hoop and pushing it away? Many players are close to 7 feet and can easily reach the hoop, so why doesn't a player just stand near the hoop and push away three pointers?
This is called goaltending and is prohibited.
In basketball, if you block a shot after it's started descending, it is ruled to be goaltending and the basket is counted automatically. There are several other circumstances in which goal tending is called as well, but they are out of scope for this question.