BIOS is massive. It implements a ton of interfaces that nobody cares about any more but they have to be there for compatibility, so vendors just shove in some code modules someone else wrote and hope for the best. Nobody writes their BIOS from scratch because it wouldn't run something and someone would certainly complain, and there's no benefit to it. And to top it off you've got this weird execution environment that nobody has the right toolchains for unless you buy them.