We've had Ctags since 1992, so being able to grep for functions hasn't been important for 32 years... but there's still people who prefer to use grep to look for functions, so having a specific keyword is beneficial for them. It may also make the lex/parse/generate cycle simpler, but I'm not a compiler engineer, so I don't know for certain whether it does.

We had ctags for lack of better alternatives, not because it’s any good

lsp can be extremely slow, on constrained devices