Many data types in scala, like HashMaps, have the postfix increment operator implemented, but Ints and Doubles have not.
Is it because if there was a postfix operator for numbers, there had to be a prefix operator too, and that would not fit the way operators are implemented in scala (as methods)?
The ++ operator is not meant as an imperative increment operator, but as a union of two collection, such that it produces new collections instead of modifying either of the old ones.
The increment operator is a very imperatively styled thing. I think (!) as you're encouraged to program in a functional style, where variable values are not supposed to change in general, it is not present in Scala.
However, this is highly unofficially what I think from a Scala programmer point of view.