Nothing like resurrecting a 2-1/2 year old thread!
First off, if there's anything that posts on KB make clear is that there's often no explanation for what turns us on and turns us off. The simplest, and most honest, answer, is because it does.
Second, "butch" lesbians conform to Straight America's stereotype of "what a lesbian looks like." So, you see one, you immediately think "lesbian!" or, conversely, when you think of a lesbian, you think of a butch lesbian. If you saw me walking down the street you wouldn't give me a second glance, and the same is true for the vast majority of lesbians. Well, except for Rosie O'Donnell.
Full disclosure: on a strictly superficial level, the "butch look" does nothing for me. But, as I mentioned above, that's just my personal preference. But to generalize about the entire lesbian community, as some have done in this thread, fails to bear personal preferences in mind.
Third, to answer the original question -- why do butch lesbians chose to dress like they do? -- the reasons are many. I seriously doubt hormones have anything to do with it, nor, to Circ's original point, does "man-hate" have much to do with it either. Part of it, I think, is what Farmer Miles points out, their refusal to conform to societal stereotypes of "what a woman should look like." Part of it, too, is what RopeFiend indicates, and exterior expression of an interior feeling, perhaps akin to the way some -- but only some -- gay men prefer to dress as women.
Finally, I've met many gay women who fit into all the different little boxes that society insists on putting us in (and which we, at times, insist on putting ourselves in). And I can say with completely that the "butch thing" has absolutely nothing to do with true femininity. Real femininity has nothing to do with personal looks or personal comportment. Yes, they "look" masculine (again, in compliance with societal norms), but they are no less feminine than any other "type" of lesbian, or any straight woman, for that matter.