A defensive player can be entering the game WHILE AT THE SAME TIME another defensive player can be leaving the game.
What we see happen instead is that a defensive player comes from the sideline very slowly. He walks over to the guy he is replacing and taps him on the shoulder. Then the player who is being replaced slowly leaves the field. The offense has been ready to snap the ball for the last 20 seconds and are not allowed to.

To your point about the rule would be changed if it was cheating, I think it probably will be changed if it happens enough and in high profile situations.

Lets say an offense subs out with 25 seconds left on the play clock in the 4th quarter of a close game in the national championship and the opponent does the super duper obviously intentional slow sub and it costs the offense a timeout or a delay of game penalty.
I bet the rule would get changed then. Those of us complaining about it are just ahead of things and you are a bit behind right now.