I think it’s fair to say at this point that the “semantic classes with reusable cascading CSS” experiment has lost to local, component-based solutions. Overuse of !important is just another sign of that.

Having “clean” CSS doesn’t have the same benefits as clean, well structured code in other areas. The underlying assumption that you can change the visuals without major surgery to the HTML is basically never true, so if you do put the time in to structure the CSS perfectly it’s often wasted when the design changes a little.

Also (as someone said in another comment) the lifetime of CSS is short compared to other application code and certainly compared to backend data structures and business logic, meaning the technical debt isn’t a huge issue. Iteration speed is often more important.