Because you don't have to throw it to him as much. That's what you all don't seem to be getting. Assume every pass goes to player A and player B. Player B will have more catches because it's taking more passes to move the ball. Player A scored 4 plays ago and didn't need 4 more catches to move the length of the field. It's because the catches are meaningless. It's the yards that move the ball.
You're backwards. You're starting with the end.
If you start with the premise that every drive ends in a touchdown, then player B has more receptions as a result of gaining less yards. You are also assuming each WR is catching the ball and open on an equal basis. That's a hell of a lot of assumptions.
But that's not the real world.
How many drives end in touchdowns each game on average? 3? 4? Your premise doesn't work in the real world.
Receptions don't reflect the end result of your premise (TD drives), they reflect the reality that:
Zero players are open on every play, and zero receivers catch every ball.
In the real world, receptions reflect how well a receiver is doing on a play to play basis.