The thinking is that users are a danger to themselves and will be tricked by threat actors to add some CA cert to the trust store, paste commands into the js console, or download and sideload malicious extensions no matter what controls are in place so therefore these possibilities must be removed.
To this day, firefox is still the only browser that prevents its users from running custom extensions without Mozilla's blessing, likely motivated by the same garbage argument.
I have very little hope for Mozilla at this point and sincerely hope that they fail soon so that better open source browsers can take its place.