Why does PPSSPP emulate the PSP operating system?

[Henrik]

I think LLE emulation could be reasonably fast on a PC at least, but the main problem is that unlike on many other platforms, very little is known about large parts of the PSP hardware, and not all parts of the kernel have even been decrypted yet (although there may be ways around that).

HLE is great for speed, compatibility is indeed a bit of a pain though, but it can be when doing pure HW emulation as well.