there pretty much is no gba homebrew
...excuse me? I beg to differ.
On my Flash2Advance Ultra cart I have tons of homebrew, including a port of Linux, a fast text editor, a calculator, Minesweeper, 30 varieties of Solitaire, Yahtzee, a very functional text reader, an image viewer, a video player, an audio player, an NSF player, an SPC player, an NES emulator, two SNES emulators, a GB/GBC emulator, a Lumines clone, a paint application, a TI-83 emulator that runs at full speed, a windowing system called GBA WinS, a word jumble solver, two 3D engine demos, and a program to change the real-time clock on my F2AU.
And that's just a fraction of all the games and demos out there. Not to mention all the great commercial games available for the system.