I've mostly had third-party contract recruiters for Amazon do this as a way of filtering candidates who can't legally work in the US. You'll end up giving them some information and then never hear back from them again.
I wouldn't even think of giving some random person my SSN. Although there seems to be a phenomenon of recruiters contacted people and then falling off the face of the earth. I think they just work by numbers. Send as many emails out as possible and hope they can find a candidate. It kind of upsets me when I get something that I'm clearly a good fit for. Then I think "maybe this recruiter actually filters before submitting people" and then they never respond when I reply to their initial contact.
I think about half the recruiters I worked with had some good jobs that didn't work out because the hiring manager they were working with had budget dry up on them. I think it's like any enterprise sales position. You end up talking to a lot of well-intentioned people without a lot of organizational backing.
Most of them talk to you to get an idea of cost and benefits so they can try to sell it to their bosses. You never hear back from them because they couldn't get any backing from the people above them. It's not that they weren't sincere, it's that there's a lack of support for their initiative, and the kind of decision they were trying to make required more buy-in than they could get at the time.
I think recruiters end up in this cycle too. The difference is that because you're the product you never hear about any of the actual sales work that goes on.
Most of the better recruiters will at least tell me the position is on hold or it got pulled. Some just completely ignore me. I guess they get a huge volume of email too but it doesn't leave a great customer service feeling.