In the 90s I used 3 different desktop OSes where the UI followed a style guide, and that style guide evolved gradually over years, allowing you to glance at a control and be familiar with its possibilities. Now even on one device, the user sees 10 kinds of different controls and new ones that are thrown at them all the time. The problem is surely that you can't rely on the user _recognising_ the control you've built. You have to design as if every user might be parsing it for the first time.
Usability has taken a nosedive with the rise of web apps. Every company wants to have their own special branding in the form of a "design system", and there's a whole industry of designers coming up with different non-standard ways to do things.
It's not just web apps. macOS has a haphazard mix of old-style checkboxes, large iOS-style toggles, newer SwiftUI smaller iOS-like toggles, old menubar checkboxes, new iOS-control-center button toggles, and a HomeKit universe of ad-hoc toggles designed for touch that make no sense for a mouse UI.
