Long history and baggage of object orientation, I suppose. There's nothing inherently mutable about dot-notation (indeed, F# uses it for accessors on immutable records as well as for "proper" object-orientation), but lots of object-oriented languages use the dot for accessing objects, and lots of object-oriented idioms involve mutation. If you're going to send a message to an object, you do kind of expect something to happen, and that's often a mutation somewhere.

Having said that, I actually don't know the history of the dot operator in any depth (https://cs.stackexchange.com/questions/89031/what-is-the-ori... suggests it arose in Simula).