@natsume_shokogami I think part of the reason is that people experience the corrupting influence of money when they get paid by their employer, so they reject money completely when working on projects for which they have enough intrinsic motivation to do them without money.
At least that’s what my emotional reaction is.
And it is sane: payment that depends on measured performance can ruin intrinsic motivation:
https://www.draketo.de/light/english/motivation-and-payment