Account says buy bitcoin - user checks history and sees 7 year old account with thousands of posts, many highly upvoted, so assumes source is sane and not astroturf.
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.
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.