I think there's a couple of different forces at play in the convergence of GUI design that we're seeing in the past 20 years. First, there's been a huge amount of widely accepted research that shows what the most accessible way to design an interface is. Things like Google's Material Design and Apples Human Interface Guidelines come to mind. Second, the widespread availability of those two specific design guides make it increasingly common for developers to just create to those. It ensures that things just work and are increasingly portable. Third, we're in a landscape where API stability and design is prized. That's partially because of the number of times that design has been broken by updates and version changes. It takes many years for developers to update their applications when a new back-end is developed, and the time in between gives broken applications, and ugly looking fallback. You can look at running GTK1 apps on modern GNOME, or X11 apps rendering on Wayland over the past decade for an example of this.

All that said, I truly miss the days when we had interface skinning. There was a skin for OS X called UNO that was absolute perfection in my eyes, and it was ported to an old version of Android back when skinning was a thing. There's nothing like it available now. Even GNOME is highly against theming and skinning now, apparently because they like breaking with every single release rather than maintaining an API/ABI and skinning support. The themes that were available for Windows XP were so much fun, even if you had to swap out DLLs to get them working.