You're assuming all rich people are pennypinchers like Warren Buffet. In reality most rich people are rich because they were born into a rich family and had the ideal opportunities to win at life from the start. The fact that in reality you won't see rich people riding buses or picking up coins on the street just shows that it's not about making most money per hour.

And in marshmallow test specifically you don't have kids who are good at business and kids who are bad at business. You have kids of rich parents and kids of poor parents. The fact that the former are supposed to act like pennypinchers, despite not ever having to worry about money or being fed seems counterintuitive to me.