1. Just because there is Linux, doesn't mean everyone should jump on the Linux bandwagon and abandon all work on illumos, NT, QNX, Fuchsia etc. Focusing everyone on Linux would kill progress.
2. Just because there is x86 or RISC-V, doesn't mean noone should invent new architectures. Apple went with their own and that's what gives them their edge now.
3. Just because there is already Emacs, doesn't mean that all editors should be Emacs mods.
4. And back to compilers, just because LLVM already has optimizers, doesn't mean that other people shouldn't explore other designs for their backends. Especially that LLVM is really slow, and a major bottleneck for new compilers now (see Rust, Zig and Jai e.g.).
If modern tires are considered reinventing the wooden wheels, sure, then reinventing wheel is the main form of making progress. But I doubt that's how people thinking about reinventing wheel, when they use that phrase to mock others of wasting energy inventing something that has already a very-well-working alternative.
I have also been told by more than one person that does professional compiler development that GCC's codebase is very difficult to work with. At least some commentary I've read suggests that this was a deliberate choice on the part of the GNU project.