It could be something to do with the following:
In high fighter lvls, you don't get any better Thac0, only hit points and saving throws rarely, ie. you don't get much when u get a lvl up, only like 3 hp. With a Druid lvl up they get spells and slots in their priest book AND Hp.
Btw I'm not including high lvl abilities
I'm also not *too* sure about this, but I would think that would be why. Oh yeah also the fighter high level abilities aren't as good as the druid/priest type ones.
Hopefully someone else who knows more will comment here as well....