I can’t really explain “why”, but the definition of who is eligible, versus who is not:
On any offensive play, the offense has to have exactly seven players on the line of scrimmage. The player at each “end” is considered to be an eligible receiver, as are all the players who are not on the line of scrimmage*.
You can get an illegal formation if one of those “ends” winds up being an offensive lineman (i.e., someone wearing a jersey number from 50-79), since jersey numbers are one way for the officials to keep track of who is eligible, and who is not.
I’m fairly sure that the reason “why” is buried deep in football history, when the rules for forward passing were first developed.
- An exception: the quarterback is not an eligible receiver, if he starts the play “under center”. A quarterback in the shotgun formation, however, is an eligible receiver.