In OpenBSD, "base builds base". That is to say, one must be able to build the system using the compiler provided in the compXX.tgz set.
gcc moved to GPLv3 and OpenBSD doesn't ship GPLv3 code. For a long time, that's the reason why it was stuck on an ancient gcc.
OpenBSD moved to clang/llvm because it has a more acceptable licence. However, clang/llvm does not support all architectures OpenBSD supports, so that's why there's stil gcc.
Users can install a newer gcc (v8.3) from packages/ports.
Newer LLVM releases are licensed with APL2.0, and that doesn't sit well with OpenBSD:
Why not? This isn't the 90s anymore. Clang 8 is a very usable c compiler with excellent optimizations and excellent diagnostics. (OpenBSD doesn't use c++, so they don't need newer standards of that anyway.) And newer versions of both clang and gcc are available as packages.
Users can install a newer gcc (v8.3) from packages/ports.
Newer LLVM releases are licensed with APL2.0, and that doesn't sit well with OpenBSD:
http://lists.llvm.org/pipermail/llvm-dev/2017-April/112300.h... https://marc.info/?l=openbsd-misc&m=147503691302850&w=2
So they're using the last LLVM release with an acceptable licence.