Comment by ameliaquining

In general, Ruby does allow mutable values in hash tables, with basically those semantics: https://docs.ruby-lang.org/en/3.4/Hash.html#class-Hash-label...

The copy-and-freeze behavior is a special case that applies only to strings, presumably because the alternative was too much of a footgun since programmers usually think of strings in terms of value semantics.

I don't think anyone likes the explicit .freeze calls everywhere; I think the case for frozen strings in Ruby is primarily based on performance rather than correctness (which is why it wasn't obvious earlier in the language's history that it was the right call), and the reason it's hard to make the default is because of compatibility.