I am sure that if we studied every single person who ever fell for a scam, we could find some people who had exhibited shocking lack of judgement even at the very early stages. I do think that there's a good chance that someone who hands over their card details immediately after receiving a message from a Nigerian prince has additional needs that I wouldn't like to dismiss as "stupidity".
On the whole, though, it is simply not how they function. People like to think it is because then they can tell themselves they're safe because they're too clever ever to be fooled; some woman on here once said she'd never be targeted because she likes to read and backpack, which is, ironically, a really unintelligent take.
There's usually vulnerability in the mix, something causing the balance of the mind to be disturbed - timing really can be everything, you could be very susceptible to something at a given moment that wouldn't normally hook you - and exploitation of traits normally considered to be good. Women who give a load of money to scammers clearly aren't gold diggers; in fact, they're often pressured to do it to prove they aren't. I mentioned a woman I know upthread who really wants to be a good and nice person and she's not stupid, she's just very susceptible to the idea of "doing good". It's rarely vanity either; women don't fork out tons of cash because it makes them feel desirable. They are groomed and often coerced into it. Some of these scams are very complex psychologically.
And so it is that if being fooled by a scammer makes you thick, it really must follow that it's even more stupid to keep on handing over cash (and time, and sex, and acts of service) to some fucking arsehole who's not even pretending to be a billionaire who's going to make it worth your while. But it's not. They are using psychological tricks and preying on vulnerability, and that's what scammers do too.