Earlier quoted context omitted.
I doubt this is the reason. The reason is that less-educated users are being tricked into installing extensions they don't want and that make using their computer miserable for them. Meanwhile, anyone that wants to write their own extension need only click a checkbox.
Then why not just make it a hidden option behind a huge "THIS MAY CAUSE BAD THINGS TO HAPPEN" warning to frighten them like Firefox do? ...because Google hate user customisability.
(This week I read about attacks convincing users to open the dev console and paste Javascript code in there. Users will do anything as long as it harms their account or their computer, it seems.)