Why has the inconsistency in the Windows user experience persisted for so long?

Although I'm not a professional developer, I have some understanding of software development. It's puzzling that Windows has never unified many clearly standardizable aspects — file dialogs still come in multiple era-specific styles, full-screen taskbar behavior differs between UWP and Win32 apps, and system pop-ups are rebuilt individually with every UI refresh.

These issues may each have technical justifications, but from a user's perspective, such long-standing inconsistencies are hard to ignore.