Highly pervasive, first step people do before starting new projects is setting up stuff like Tailwind, Shadcn; they also don't bother much with modifying how it looks since it looks decent out of the box causing similar looking websites everywhere; similar to how the Bootstrap craze was back from 2012-2015/6; where all websites just looked the same[0]
A lot of the sites that use shadcn's default styling are AI-generated. The people that use it in hand-written code usually customize the appearance at least a little bit.
[0]: Example of the common "Bootstrap style" https://getbootstrap.com/2.3.1/assets/img/examples/bootstrap...