it is a UI designed to be hard to use
1) UI = a clearly documented way to configure all features and make the software work exactly how you want.
2) UI = load a web page and try to do the thing you wanted to do (in this case communicate with some specific people).
FB is clearly terrible at 1 but pretty alright at 2.
Then people expect accounting software to be just login click one or two buttons.
That's just a stupid limitation and not even a technical one. You could happily send GBs over email. You can also easily filter allowed attachment size by sender on the recipient side, because by the time the attachment size is told, both information was already provided.
Commenting on things is from a list of features (to be distinguished from UX/UI) I talked about.