I think it's actually very simple... the paradox of choice. You introduce somebody to your attractive single friend there's a real chance they hit it off and form a relationship. You introduce them to 100 attractive friends, one makes more money, one has a more stable job, one is flirtier, etc then it's both exhausting and none of them seem like a great opportunity. I think there are certain basic psychological facts…
> one makes more money There's this joke about a man in his 40s. He goes to the gym and asks the instructor which machine he should use to look more attractive to young ladies. The instructor looks at him and says "you'd do well by using the ATM outside".
(Certainly there's a baseline: most people want a partner with at least the financial resources to take care of themselves.)