Because it doesn't follow thatMaybe not in the real world, but this is a fantasy world that we're talking about. I'm not going to talk about overtly magical things like dragons and fireballs. It's a world that has apparently non-magical creatures that shouldn't exist in the real world - things like giant insects, hill giants and purple worms. So why not apparently non-magical martial daily abilities, as well?
a) D&D world has magical things
b) fighters are in the D&D world
c) therefore fighters have muscles that magically become tired independently
d) i want (c) to be true soooooooo badly because of the Daily mechanic that I'll say anything to justify it
I dislike it.. a whole lot. I think it's lame and uncompelling enough that it completely fails to convince players who currently dislike dailies (for in-game as well as metagame reasons).