When successfully adding a suggested attachment from family search, a source date is automatically added.....except the attachments for find-a-grave. I am constantly backtracking to input that source date for myself and for others.
The reason Find A Grave (and Billiongraves) citations do not automatically have a date is that it is not necessarily clear what that date should be for these types of sources: date of death? date of burial? date of cremation? date of memorial service? date of obituary? date on gravestone (even if it's wrong)?
In the default date sort on sources, undated citations go to the bottom of the list, which is generally where death/burial sources belong anyway, so I mostly just choose not to worry about it.