I don't like the idea, because a + b should produce new list c without modification of both. Which is not memory optimal and cost of it is not obvious. Also dict is used a lot for sub classes and that could break a lot of existing functionality with potentially no benefit for most of the developers. I don't think that merging is very common operation for dicts and even so it could be done with 1 or 2 update function calls, but that will be obvious in that case, while '+' in deeps of duck typing code is not. Also absence of '+' operation for dicts is kind of guard for type validation in case if someone passed dict instead of integer. Which is pretty common when you parse some JSON from client.
It does produce a new list/dict, etc. That's what you want to avoid side effects in many cases. Modifying is actually an exception. Handy to have, but not common. Even numpy arrays recreate everything every time.
