Use the Profile URL field and customize their Profile URL as needed. Set it required during registration and they won't have a choice but to supply an SEO friendly URL. Otherwise usernames in general are not very SEO friendly making it hard to reverse them, do a database query to match the user, and utilizing the user id from that match; that is why the user id is prefixed. See our demo site below with perfectly fine URLs.
demo.cbdemosites.com/profiles/susan
demo.cbdemosites.com/profiles/demo