I'm not sure that's always the case — we don't expect assembly to be a high-level language after all! The more specific and batteries-included the browser becomes, the harder it is to go off the beaten track. My standard example here is date pickers — theoretically it's just a simple component, and yet there is no one-size-fits-all option. What works for booking an appointment won't work as well for putting in a date of birth. What works for a date of birth won't work if you're trying to book a set of nights in a hotel. You might want to include prices for individual days directly in the date picker. You might want to show which days are valid and which aren't. You might want to show several months, you might want to show just a week.

I don't necessarily disagree that more components in browsers is a bad thing (I've been very happy to use the new modal element, for example), but I think the browser is working better right now as a lower-level (albeit still fairly high-level) platform that allows people to build a variety of documents and applications on top.