Because once you dole out the receptions to TO, Witten, Crayton... the backs, plus the reserve TE's then there isn't much left for the other WR-- even if that wide receiver is the #2.
You bring in a Roy Williams and you take receptions away from three guys you just doled out decent to large-scale contracts too:
Owens
Witten
Crayton
How many passes do you think this offense is going to throw?
This offense had 342 completions last year, with last year's #2 receiver catching 50 balls-- 50 divided by 16 = 3.125 balls per game.
And that's without having a terrific receiver coming out of the backfield in Felix Jones and that's without the apparent progress of Miles Austin and hopefully Sam Hurd.
Again, I say, who you gonna take receptions from in order to justify a Roy Williams and what makes you think the people you take receptions from aren't going to raise heck over it?
And personally I don't know if this offense even completes 342 passes again this year-- I think the number will be lower, with an increased emphasis on the running game. So now the "pie" could become even smaller.