Mostly frameworks that follow guidelines like Material-UI, the insane aspect ratios and resolutions, and then features like Dark/Light mode.
It is the worst user interface design I have ever been forced to use in my life. It's like they designed it for children or something. And from then it spread to general Web design and to software including GNOME, with GTK3 and the Adwaita theme. Suddenly everything became low contrast and low density, with oversized buttons and excessive whitespace.
That part is mostly because of smartphones and their small touchscreens. You don't really have a choice with these fat fingers.
And for consistency purposes for multiplatform apps/webpages, desktop interfaces followed.
The unfortunate thing is that we have had smartphones for 20 years and no one could develop a paradigm with good desktop and mobile ergonomics. UIs improved massively from the 1980s to the 2000s, but from the 2000s to the 2020s, there is essentially no improvement. I understand the disruption caused by smartphones, but not the lack of progress. If anything, modern UIs are objectively worse than they were 20 years ago. Look at that mess that is Windows 11, nothing is consistent, even when you only consider what is shipped with the OS itself.
I think it's probably a marketing drive to cater to as wide an audience as possible, to the lowest denominator of user. They probably made a rational decision to do this because it would increase revenue by simplifying the interface. So these devices went the way of TV in the end.