But because Intel (mostly) sells commodity parts, any effort improving compilers has a limited ability to benefit Intel without also benefiting competitors (i.e. AMD Ryzen/Epyc). Compiler improvements also benefit owners of existing devices, which reduces the pressure for customers to trigger a hardware upgrade cycle. However you cut it, improving x86 compilers doesn't improve Intel's profitability in a competitive x86-dominant market.

Apple doesn't have any such conflicting motivations because they sell the product rather than commodity parts. For Apple, improving the compiler as a unified part of the Apple Silicon development process means they can sell a better product with the same unit cost, with only residual benefit to competitors that don't have access to Apple Silicon hardware.

Intel is a significant contributor to GCC. Don't know about clang.

All the Intel development tools have been freely available to everyone for a while now... A large part of them are based on Open Source components like gcc or llvm, and most modifications have been contributed upstream.

It's the packaging of all tools and libraries that Intel provides, extending to much more than "a compiler".

?? https://github.com/intel/llvm

sure but Intel definitely contributes to both GCC and clang. Hell, GCC is the main compiler they use for their own performance-oriented distro.