that's where a large chunk of executive compensation comes from.
and the answer to your question is market forces. if company A pays its executive only in company equity, while company B pays similar compensation but half stock and half equity, executive will choose company B because his income has less variability.
company A will attract less talented executives compared to company B, and management has a lot to do with a company's success.
the problem with executive compensation has less to do with the form and more to do with who sets it. the market for execs is a long shot from the more competitive labor markets that you and i (non executives) face, and executives have a lot of influence on their pay, while we don't. for executive compensation to be more reasonable, there needs to be more pressure from shareholders and the general public to keep these costs down. there used to be a lot of political pressure for companies to keep executive pay low, but that pressure disappeared about the time we started voting for movement conservatives, starting with reagan and perhaps ending with bush.