However, older systems were also much simpler. The GBA BIOS functions may have been simple enough that it was not difficult for the emulator developers to reverse engineer and reimplement them natively. Newer systems such as the PlayStation no doubt had extremely complex software controlling hardware like the disc drives. I've even seen references to C standard library functions in the debugger documentation and many notes detailing how buggy they were. I have no doubt there are games which rely on those bugs. Reimplementing all this software would be a significant undertaking. Easier to just make the user obtain a copy of the original software. That's how its done to this day even though the PlayStation is already 5 generations behind.

It's definitely possible to emulate or reimplement this software, it's just hard and the focus of emulator projects is usually on the hardware. There are other projects which aim to do this stuff though! I've seen game engine emulators which are open source reimplementations of the engines of older games. Users are expected to plug in the copyrighted game assets themselves. Perhaps one day consoles will be nothing but standard PCs under the hood and emulating them will consist of reimplementing the APIs the games use to interface with the operating system just like Wine and the Windows API.