No reason, plenty of private companies do that, but they usually stay on the smaller side, so you don't hear about it. Valve (Steam) is a prominent example. If you have the chance to work for one, take it, good chance you'll be treated well. It's when companies are publicly traded and sociopathic CEOs take the reins saying 'I have an obligation to my shareholders' and 'line must go up' that things truly go to shit. So, some combination of putting psychos in charge, the greed of capital and a lack of regulation IMO.