It just doesn't scale linearly. There's substantial evidence that happiness scales logarithmically with income.

https://www.astralcodexten.com/p/contra-pritchard-on-liberal...

I don't know, self-reported happiness seems like an awfully unreliable metric. For one, I've noticed that projecting unhappiness is much more of a faux pas in rich circles. On its own this could account for much of this relationship.