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.