Customizable look and feel means lack of consistency, and confusion as to why things look different without good reason.
We will never live in a world where an abstract concept like a checkbox can have an 'assigned' specific visual affordance that doesn't allow for adjustments. Such thinking is stuck in the past and won't allow for new/better UI paradigms.
Everyone, literally everyone is running their own thing. Name me big company that doesn't roll their own design system.
Preaching purism really doesn't help anyone, let's just accept reality as it is, and stop chasing some utopian dreams that just won't ever materialize (and if only because it would make a lot of jobs and professions useless, overnight, and there's too much inertia for that to happen).