Also like the act of creation. I get ideas in my head and want to get them out. In high school and college, my homebrew was a ring world. Whenever I'd make up a new world, I'd just move it over 10 or 20 thousand miles and sit it next to my old world. Teleport transportation between "worlds" and flat maps were some of the advantages. Caused some interesting problems with things like moons and such but those were easy to get around.
Another reason is so that I know the world. It all fits and isn't a patchwork of different styles and such and i get into it.
A third reason, is because then the players are depedant on me for information. With published systems, it's ineveitable that other players will also be DMs who have or even run games in those published settings. Then their out of game knowledge gets too mixed up with in game knowledge and hard to get out of player's heads. If my world doesn't sync exactly with the published setting, then they get confused and can never remember the changes I've told them about in history and setting. With a homebrew, there's nothing to confuse them and none of the players are going to know the official history better than the DM.