I don't know whether this question has a clear answer, but I was unable to find anything about this on Google, so here goes.
Despite the emergence of "rich" document classes like KOMA-Script or
memoir, it is my impression that a substantial amount of LaTeX users still uses the standard document classes (correct me if I am wrong). This is indicated, for instance, by this answer to a TeX.SE question. For researchers looking to submit their work to a journal or publisher, there are actually (somewhat) objective reasons for sticking to the standard classes, since many journals have their own classes and the conversion process is typically the simplest when starting from a standard class. To summarize:
Many people still use the standard classes, and there are sometimes good reasons to do so.
At the same time, the standard classes have not been changed for what feels like decades (again, correct me if I am wrong). My gripe with this is that many of the common issues with the standard classes actually have solutions which would only require "micro-updates" to the classes themselves. For instance, in the
book class, a common problem is the uppercasing of page headings (which is hard-coded for things like ToC or bibliography) and the lack of horizontal ruling capabilities below the headings. Sure, you can use
memoir, or KOMA-Script, but why is there no update to the
book class which makes this stuff customizable? (This would probably amount to changing less than 1% of the class code). To summarize again:
The standard classes have been unchanged for decades, yet some of their issues could be fixed with very simple modifications to the class code(s). Why is this possibility not being pursued?