It's an interesting question, why immutability is not built into more languages as the default, so that the most intuitive syntax of assignment produces new values.

Without having any expertise in the matter, I'd guess that mutability has the advantage of performance and efficient handling of memory.

  obj.foo[5].bar.a = 2

An immutable interpretation of this would involve producing new objects and arrays, moving or copying values.

Another possible advantage of the mutable default is that you can pass around references to inner values.