Because they're supposed to take the heat so the board can replace them and say the company will change course now that be bad executive is gone. It's like an unwritten rule. Though in recent times, even that hasn't happened that often.

Second part is that it's much easier to make unsavory decisions when you get paid a ton. They usually get paid most of it in stock options, so they have to raise the share value to benefit the most. Who do you think is more likely to make immoral decisions to boost the company's short-term value, the guy getting paid 200k, or the guy getting paid 5 mill anyway + anywhere between 0-50 million in stock options depending on how well they the company does?