EFI is a specification, not an implementation, where the core pieces are still controlled (And _never_ opened up) by vendors and is usually still a big wad of real mode assembly that nobody wants to touch. There is no 100% open-source EFI-compliant BIOS implementation. The specification alone for EFI is over 1,000 pages.
To top it all off, to even begin development on stuff like Tianocore you need to agree to draconian licensing terms such as: "You acknowledge and agree that You will not, directly or indirectly: (i) reverse engineer, decompile, disassemble or otherwise attempt to discover the underlying source code or underlying ideas or algorithms of the Software; (ii) modify, translate, or create derivative works based on the Software; (iii) rent, lease, distribute, sell, resell or assign, or otherwise transfer rights to the Software; or (iv) remove any proprietary notices in the Software." (
https://www.tianocore.org/nonav/servlets/LegalNotices?type=TermsOfService ).
So I guess your question is sort of like asking why people don't like to use proprietary drivers, even though there are some out there that work very well. The nice thing about Macs is that Apple seems to have went out of their way to make EFI invisible to the user. I don't trust that this will happen on most other pieces of hardware. The BIOS belongs out of the way, IMHO.