Why homebrew?
In my case anymore, that is almost equivalent to "why breathe", but we'll pass that.
When I started with those three little books way back when there really wasn't much of an option. There
was not campaign that came with the original books and by the time the Greyhawk material started coming out it simply didn't fit with the style of play I had already developed with my group.
Over the years I have used (or at least attempted to use) multiple pre-packaged settings. Some were easy to use (Alpha Complex, Al Amarja, both probably because they were purposely vague), some were a bit harder but worth the effort (Star Wars, Star Trek, World of Darkness, Mythic Europe -- they had to become "our version" of the settings), while others were harder to get into (Greyhawk, Forgotten Realms, Theah -- in all cases I ran into folks who told me how "wrong" the version I had was).
In the end, I have found, since I don't do convention gaming often and that the players I have really
like contributing to the scope of the world, that homebrewing offers a great freedom. Not only do I have, as the GM, the ability to craft a setting, but everyone at the table has an emotional investment in the setting itself, something beyond simply their characters.
I am currently in an amusing situation. I have an older setting, New Mavarga, that I first cobbled together under 3e. Of my current four players, three were in the previous incarnation; they asked that I bring the world back, but under a more appropriate (to our eyes) set of rules, so we have reworked it using
7th Sea; we toyed with the thought of using
Swashbucklers of the 7 Skies, which also seemed to fit the mood, but it felt just a bit "off". The point here, however, is not really the rules set -- it is the fact that out of all the pre-published settings out there, out of all the worlds the players could name, they specifically requested, nay
demanded, that we return to a setting that I had created in the past.
That, as a GM, is high praise.

I live for moments like that.