Because not-for-profits, which have special legal and tax treatment on the theory they're doing good for society, are accountable for how they spend money in ways that for-profit companies aren't.
I also think it's hilarious that "don't overpay executives and instead spend the money on the good you're supposed to be doing" is an "extreme ethical standard". How did the Overton Window get moved all the way to the basement?
It's not just executives though. It's all staff. It's hard for a lot of people to take rather huge pay cuts to work at a non-profit.
Very few people at non-profits are "overpaid" when compared to salaries at a similar for profit company. Non-profits also have less tools available to pay their employees, such as stocks.
I also think it's hilarious that "don't overpay executives and instead spend the money on the good you're supposed to be doing" is an "extreme ethical standard". How did the Overton Window get moved all the way to the basement?