Because it renders acceptably in common browsers, and that's what actual users care about. If a website does not validate, only an extremely tiny subset of user

Because it renders acceptably in common browsers, and that's what actual users care about. If a website does not validate, only an extremely tiny subset of users who test websites in validators is going to care about it. Google's home page is now huge with the javascript and Google+ additions - but it used to send invalid HTML missing end tags on purpose. Browsers still render unclosed tags when reaching the end of the response, and therefore they could send a smaller, faster HTML page by leaving off closing tags.