That page is probably among the top 3 most analyzed, optimized and A/B tested webpages in the world.
Google controls the browser, the page, the CDN, AND to a large extent the very standards that the browser has to comply with.
If what you claim is true, with all of this authority, why can't they write a compliant web page?
It could be a way to keep some kind of competitive edge or some kind of fingerprinting strategy or for some other reason altogether.
I'm not sure why you'd think that they "can't" write a compliant web page. It's obvious they can, just like it's obvious they've been paying a bunch of experts top dollars for multiple decades to think about and test what exactly to write in this page's code. It's also obvious they've taken into account the basic fact that every character* they add costs them a measurable amount of money to serve given their scale.
It's therefore pretty obvious that they're deliberately choosing to write a non-compliant web page. Presumably because among the multiple billions of users they serve this page to, a high-enough-to-matter portion is still using old and/or non compliant web browsers and they don't want to cut them out.
* past certain packet length cutoffs
They even ship Chrome with their applications, because people can't be bothered to learn neither native UI frameworks, nor portable Web standards.