Ignoring these frameworks stop third-party access.
They go against the principle of reusability as they are all unique.
This means the browser can’t cache or reuse styles that should be and are identical, because each instance has a different name.
CSS was designed to allow one style to be used many times with the same class name.
These frameworks take that logic and all the associated benefits and throw them out the window.
If there is a common button design used ten times, these unique IDs mean it is seen as ten different buttons, wasting time and resources.
All because the designers of the framework were lazy and the user of the framework didn’t do any due diligence.
As a 35 year veteran of commercial software design with a good chunk of that in web design (plus desktop, mobile and massive installations) I have more than a little experience. I designed, wrote and sold a CMS that let users add content and let the CMS do the complicated bits.
It was written in a programming language I wrote specifically designed to create variant CMS solutions that merged the front and back end work with DB integration that could add any sort of data or content by essentially answering a few questions.
All CSS was compliant and reusable, all JavaScript was pure, no third-party libraries, you could change a setting and it would support one of many server-side scripting languages, another parameter determined the database it would use. If changes were needed you simply made them and compiled (technically transpiled). If a client needed to shift server that had different tech we processed the site again and uploaded elsewhere and all data remained as was cross-graded.
The reason I mention this is, if a small company such as mine can do things correctly and easily, why don’t the big companies?