Because of Bootstrap. Everyone uses it. Every old website that looked different in the past, now does a UI refresh (by just using Bootstrap out of the box with minimal customisation) and becomes another one of those the same looking websites.
Promethease is a good example. Years later a bootstrap website signals to me "it's about the functionality, not the design". https://promethease.com/
Before that everything was jQuery UI. Government websites still use a ton of it. All of Colorado's web portals are built with it. And now things are shifting towards UI frameworks "plus more", like Ionic, which adds a bunch of design/devops workflow and cross platform functionality.