There is people with a high level of trust. They will believe almost anything. Sometimes they are in a vulnerable situation, sometimes it comes with age as cognitive function gets worse, other times maybe people with high stress situations looking for a way out of their problems, they exist.
In phishing, the scammer knows that most people will ignore their mails. But, they are trying to get to the vulnerable people, the ones that will bite the bait. "significant numbers" is difficult to evaluate. Maybe it is a very low percentage of the people that is targeted, but when the targeted are in the millions the victims are going to add up.
And, in my experience, this is a psychological trait. A kind of opposite to paranoia. A paranoid person will not believe anyone. A too-trusting person will believe everyone.
Phishing and other scams prey on the vulnerable.