> Wouldn't it be easier to draft a set of design principles for the project?

Isn't this what a framework gives you out of the box? Hiring should become easier if you can simply hire another Angular dev to join your large team. I'm assuming that your in-house design principles won't be as thoroughly battle tested as something like Angular or React is which has received tens of thousands if not millions of man hours of testing and refinement over the years.

I'm not really advocating for or against, since I've never used a large framework like this in a team environment, but there must be benefits. Lots of smart people aren't using it because they are dummies that can't do better.