Ask HN: Why do the Big names of the Internet dont have a valid HTML Markup

I have been coding for a while (as a Web Developer). And I m really concerned about following the rules/ best practices for coding. I always check my HTML markup with the W3C Markup validator, and try to minimize the errors as much possible as I can. Almost try to make them zero (for my satisfaction). But I just checked up for the Big names of the Internet for their markup (including Google & Facebook). And I was shocked to see the number of errors and warnings their markup has. I've learned (has been taught) as one of the major rules in Front End Development that your Markup has to valid as per the W3 Validator. So is that so that the Big Internet companies just don't tend to care about any of the HTML Standards or so ??