The rich are just suspicious of the poor. They trust people from similar backgrounds as themselves. Employers irrationally think they can gauge a person's trustworthiness just by interacting with them for a few hours during an interview. And trustworthiness is virtually impossible to test for in higher paying roles: people earning low wages ususally don't have the power to defraud the company too much, but there's a lot of people with that limited power so they're a threat. Wealthier employees could engage in petty fraud too, but they have less incentive to do so because they don't need the money as much, and there's not as many of them, so even if they did it would be less damaging. Instead the concern over richer, more skilled and educated employees is that they'll steal large amounts from the company, and unless they end up in prison, nobody will ever know (contrast with working-class fraud which will usually only end up losing the person their job, so there's less incentive to avoid being caught).

There's probably no conspiracy, just generalized classism.