I think people didn't know what they were voting for, in detail anyway. I don't recall EASA being mentioned by the mainstream media during the referendum campaign! If anyone had thought about it, they'd have thought that a sensible arrangement would be made and the UK would continue to be a member of EASA in some way, just as the UK would continue to be associated with various other EU agencies if common sense was followed. The negotiations aren't finished and there's still time to sort out an arrangement, if there's goodwill on both sides.