It’s a risky blanket statement that causes bloat by including all the mistakes you made while building the original product.

It would be tiresome - and bloated - to include a class pass-through for every component or assigning custom properties (from the RFC linked) for all potential properties on every component, just in case it's gonna be used in layouts that requires it. Wrapping them in a wrapper div is certainly an option, but potentially creates 100s or 1000s (long lists, several lists etc.) of new elements in the DOM slowing down low-end devices.