When applied to the web, maybe, yes. But then there are desktop apps, and entire operating systems, designed this way.
I think it has something to do with the phones after all. Everything is "mobile first" these days (to a ridiculous degree — there's a large grocery delivery service in my city that is ONLY available through a mobile app, so they lost me as a customer), so designers have much more experience with touchscreen UIs. Desktop anything feels like an afterthought oftentimes. It's just easier to take a mobile UI, blow it up to a computer screen, or put it into a window, optionally add a dual-pane layout, and call it a day.
That, and the fact how many designers regard the result of their work as a thing in itself, an art piece to be admired, instead of merely a coat of paint on a tool that just helps people get their f-ing stuff done more efficiently and then gets out of the way.
I think that also has to do with "more white space = more scrolling = more ads".
Reddit is a perfect example of this. When they "updated" their UI to the current one, they added a ton of extra white space which forced people to scroll longer, thus spending more time on the site and viewing more ads. They also use a ton of other dark patterns.