Economics doesn't assume everyone wants to maximize money. It assumes everyone wants to maximize utility, and then uses money as a (poor) proxy for utility. It's a subtle, important difference.
Considering that, it's clear why we'd use money as a proxy. Not a perfect one, of course.
Labor supply theory, for example, assumes people maximize utility by choosing a combination of work and leisure. It doesn't assume people work as much as possible (or, maximize their money).