It actually is possible to create a bio section on your user profile, but it requires jumping through some hoops. Specifically, if there's a page for User:yourname on the wiki, the main site pulls it in as a profile and it becomes your main user page (you can still get to your comments by clicking "Comments" or "Overview"): see for example Gwern's user page on the main site, and on the wiki.

Most people don't bother, though.