This is more of a system issue than bad behavior of an individual charity.
Charities can underpay a little bit, because working for a charity has its own appeal. But if you want a talented, experienced person to run your org, you have to consider what they could make if they worked for someone else. San Diego is not a cheap city, and has its fair share of CEO positions.
If you really want to stretch your dollar though, local food banks are probably a better bet.
He already knew he could. He budged it during Avengers Age of Ultron, and he probably would have noticed it move (Thor did). But he stopped trying so he didn't embarrass both himself (by claiming to be worthy of godhood when he's a pretty humble guy) and Thor (by being just as good as him despite being mortal).
So, when the opportunity came again, it was a no-brainer. He knew he could, and he knew it'd be useful to him.