Apple doesn't really care about C++20. Their LLVM efforts are focused on Objective-C and Swift.

C++ as used on Apple platforms is mostly related to the Metal Shading Language (a C++14 subset) and IO/Driver Kit (an Embedded C++ variant).

Google has their style guide, only recently updated to C++, isn't a compiler vendor and apparently losing on the ABI vote has made several employees move away from their clang involvement.

Then the other vendors like Intel, Embarcadero, IBM, Codeplay, Sony, Nintendo, ARM, NVidia, AMS have their own agendas and do not upstream everything.