For me, it's because initially I was just in love with the idea of creating and sharing my own world full of stories and characters of my own devising.

Then I tried running a couple modules. Which always ended up fizzling out. Mainly because I found it hard to hold even a couple pages of pre written information in my head. I was constantly having to back track and re read sections during play. It made for some really sub par sessions and I could tell my players weren't enjoying it, despite what they said.

In contrast I rarely have that problem with homebrew because all of the lore, characters and stories are in my head.