Why do mainstream OO languages not have immutability on class-level built-in? [duplicate]

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• Complete immutability and Object Oriented Programming 7 answers

I regularly write classes which can only have immutable instances, much like string.

I am wondering why Java or C# or VB.NET don't have immutability built-in into the language? That way, I can communicate immutability better to my fellow programmers and also make assumptions about state in the class.

For instance: immutable class Foo { public string Name { get; set; } //compile error: mutable property on immutable class }