But if you mean that they cannot be easily returned from functions, the issue is that mappings can be massive. Well, they’re accessible if you have access to a node and can inspect storage.
You can also find a robust implementation of EnumerableMap in OpenZeppelin. There’s even an example of that in the docs: Iterable Mappings. Implementing an iterable mapping is pretty simple. The simple way around it is to store the keys separately if you actually need them.
Mappings are relatively cheap because they’re not implemented as a complicated data structure but instead treat the whole storage as a massive array where collisions are unlikely as long as the keys you calculate are random enough. This is an inherent limitation of how mappings work.
