Competition. If all your competitors are doing something that you want to do but know will piss off your employees, you can safely do it too and know that they don't have anywhere else to go. Most of these big tech companies operate in markets with huge barriers to entry and very differentiated skillsets; a new competitor isn't going to spring up overnight without some form of warning.
Most hired executives play not to lose rather than playing to win; the nature of their compensation packages incentivizes it, where big wins accrue largely to diffuse shareholders while big losses mean they lose their fat executive pay package. It takes a founder-CEO to play to win, but if the hired CEO had that skillset and that inclination, they'd be a founder rather than a CEO.