Dumb question: is there a reason to make a gameboy emulator besides interest? Is it not a completely solved problem space, with all games playable with optimal performance?
In the case of emulators I suspect that many developers nowadays never went underneath the syscall layer (or even beyond the interpreter) so if they're interested to learn about how it all works deep down it can be a good challenge. Of course the GameBoy is archaic in most ways but it's a start.
It's more like building a car in the garage. There's no real need to do, but some people enjoy the process of figuring out how everything fits together.
Emulators of other platforms tend to need tons and tons of work to even get to booting.