http://v236xhqtyullodhf26szyjepvkbv6iitrhjgrqj4avaoukebkk6n6syd.onion/t/why-does-tor-browser-use-private-browsing-should-it-change-to-normal-mode/8802
It is a huge comfort problem to not have open tabs or history, as this also saves the use of search engines or way too many bookmarks. And even if this was opt-out and private browsing disabled but everything cleared the same, users could easily change the settings. If they would do this while private browsing was the default, their fingerprint would be way different.