We used to use ARM proprietary C compilers at my last job doing firmware. Every time a compiler bug was fixed in one version to the next a new one was introduced. So we ended up sticking with a particular version that worked well and keep evaluating new ones till a better one is found. And we paid tons of money for the compilers and support contract back then. Maybe the situation is better these days.
Apple/Intel/Microsoft may have lots of money, but that doesn’t mean their C compiler teams get much funding.
I hear people say Apple’s work on clang is limited to their needs, and those may not involve getting full C11 support.
Microsoft also may not need full C11 support for their internal use. That can make getting that a low or zero priority task.
Intel recently moved their backend to LLVM. That doesn’t give me confidence they’re investing heavily there.
Now, gcc technically is volunteer work, but lots of development is done by people paid to do that by their employers.