I wonder if the idea is that adding pseudo-classes for the different things is not really scalable because you would ultimately need to do it for a lot of things -- requiring browsers to implement, test, and maintain custom code for each selector -- and you cannot anticipate every website's needs.

That would be a better framing for this feature. Given that framing, I can now see how it could be useful for things like custom controls not present in the set provided by the browser.

It would be interesting to see if browsers could use this mechanism internally to implement pseudo-classes.