I don't think that it has anything to with ethics.
Private industry pays a lot more and, like it or not, money is a key motivator.

Since when?

President: $400,000 includes a $50,000 expense allowance.
Vice President: $237,700
Rank and file senators and representatives: $174,000
House and senate minority and majority leaders: $193,400

I'd say that those are pretty hefty salaries.