The major issue, IMO, is that the DOM is just markup with an API grafted on, and so any framework attempting to add the slightest bit of composability or state

The major issue, IMO, is that the DOM is just markup with an API grafted on, and so any framework attempting to add the slightest bit of composability or state management has to twist and turn itself to be compatible in all browsers. And that's how we got here, because everyone wants to make their applications available on as many platforms as possible, without having to maintain five or six separate teams that have to coordinate a roadmap, and the browser is the only somewhat consistent way to do this.