Basically anyone who knows your Gmail address can spam you with these requests. It's awful because I so rarely get Drive share requests, so I keep notifications on because if somebody actually needs to send me a legitimate file, I'd want to be alerted to it. And there doesn't appear to be much in the way of spam detection on this system.

Though that might have changed recently. I used to get these very frequently, but I don't think I've seen one for probably a year now. Maybe they finally started detecting spammy share requests? Or maybe I've just been lucky for a minute.