Because it wouldn’t work for every website and the web almost never breaks backward compatibility.

It would work a lot better than what mobile browsers are doing now. The only websites that can be argued to be helped by that were those with fixed table layouts - but those are still pretty unusable even with the standards-noncompliant hack. And websites that were mobile-compatible before like plain HTML sites like the TFA are needlessly broken.

So if I have a block of totally unstyled HTML, why wouldn't a phone browser just display it in the most sensible way possible?