Because most employees assume money is a given, and they align with a mission instead. It is also not socially acceptable to be in something just for the money unless it is wall street.
I'm not sure if I can agree with that. In general that may be true, but I would have to say most people do something they don't like 'just for the money' at some point in their life. Something like working as a kid for minimum wage in a store. It's most likely that you are just working there for money.