If you flip the setting, that doesn't solve a problem of a user installing the browser for the first time, installing an extension like Stylish †, going to a porn site in private mode and getting their private browsing history leaked to the extension operator together with their regular history. Who thought that is a good idea? :)

† Stylish is a … special case [0]. But see also hypothes.is or any other extension that relies on a "user-created content store".

[0] https://robertheaton.com/2018/07/02/stylish-browser-extensio...

They are always following chrome's tail.

But anyway, the reason probably is that some extensions are abusive of personal data and you don't want them sniffing stuff in private browsing (with out your persmission).

It's not perfect, but there are prominent prompts about this when you install an add-on for the first time and when you open a private window. Screenshots at https://imgur.com/a/d4ZnAHI

No one thought that was a good idea. The good idea is to prevent add-on authors from stealing private browsing history.