[pluma]

Still playing catch-up. React is already on 16.

Joking aside, apart from semver another reason for the weird versioning in Angular is that it's a rather large collection of modules, especially if you include things like RxJS (which technically aren't exclusively part of Angular but affect APIs due to their widespread use). So it's fairly easy to get a semver major version bump because of an implementation detail you likely wouldn't care about but that breaks backwards compatibility with some older library in some way.