Imagine the problems that would cause! You have Chrome 58 Beta, and stuff works one way. Then they say it's good and release Chrome 58 final, and all of a sudden, stuff changes all over the web.
UA string is just one example of unfortunate hacks that evolved in the web protocols. Compared to probably everything else in HTML it's probably just not even worth it to consider fixing it. We'll always need the old string for compatibility, so it's really only to save a few lines of parsing. Compared to the nightmare of parsing rules for HTTP and HTML, it's not even relevant.
UA string is just one example of unfortunate hacks that evolved in the web protocols. Compared to probably everything else in HTML it's probably just not even worth it to consider fixing it. We'll always need the old string for compatibility, so it's really only to save a few lines of parsing. Compared to the nightmare of parsing rules for HTTP and HTML, it's not even relevant.