Speaking from a React perspective at least, I think it's not the default because technically nothing stops someone from writing really smelly, non-idiomatic cod

Speaking from a React perspective at least, I think it's not the default because technically nothing stops someone from writing really smelly, non-idiomatic code where memoizing it could cause logic errors. At the very least it would be a breaking API change I'm with you, I think catering to those cases is a fool's errand, but I understand the argument for doing it.