Because the Fighter class has no actual class features
This. Anything a fighter does can be done by another class with the same feats. Weapon Specialisation does not count as a "thing".
@Sstrad: If you like the Fighter and got access to some sources and time... Why don't you make a fighter handbook? Would be really nice
Ofc, it's a lot of work and won't be like a way to uber-over-powered builds, but it would help a lot of people, I'm pretty sure.
It's more... the fighter class is too generic for you to decide on it before you decide your character concept (every class "fights"). There could be, say, a TWF handbook with fighter builds in it, but a handbook for fighters wouldn't really make sense. It's like a cookbook of recipes that use salt.