That's my point though: how often do people do that check? I don't think I personally ever have. Maybe I'm just weird though? I tend to not just go with a single user/comment/thread, but rather try to find multiple different answers and gestalt from there, so individual trustworthiness is less important.

You have to remember that with spam, they're looking for gullible people, and the marginal cost of the spam activity is extremely low. To some extent, the Nigerian prince scam works not in spite of, but because it's nonsense.

Obviously it's higher than zero, else the market for accounts wouldn't exist.

Isn’t this begging the question though?

I.e., the thing you have just stated is exactly the thing we’re trying to figure out.

No, because we're talking about a phenomenon that indisputably exists.

there are definitely browser extensions that will call the API to check account age and highlight suspicious accounts, that's how "checking" generally happens except in egregious cases.