A problem with Explorer, that it also shares with macOS Finder[1], is that they are very much legacy applications with features piled on top, and Explorer was never expected to be used for heavy I/O work and tends to do things the slower way possible, including doing things in ways that are optimized for "random first time user of windows 95 who will have maybe 50 files in a folder"
[1] Finder has parts that show continued use of code written for MacOS 9 :V
This blows my mind. $400B in annual revenue and they can't spare the few parts per million it would take to spruice up the foundation of their user experience.
This is speculation based on external observation, nothing internal other than rumours:
A big, increasing over last decade, chunk of that is fear that they will break the compatibility - or otherwise drop in shared knowledge. To the point that the more critical parts the less anyone wants to touch them (heard that ntfs.sys is essentially untouchable these days, for example).
And various rules that used to be sacrosanct are no longer followed, like the "main" branch of Windows source repository having to always build cleanly every night (fun thing - Microsoft is one of the origins of nightly builds as a practice)