All apps I'm using adjust colors automatically - except the Geany editor widget. Most websites I visit regularly adjust automatically, too.
I would expect your browser does not turn pages dark when the desktop theme changes to a dark theme, it doesn't here.
AFAICT many people want to keep the current situation since they grow used to the colours semantic meaning, its not just coloured to be pretty, the colours have meaning, eg "all types are yellow, strings are green etc" and they don't swap light/dark often enough. So as @codebrainz said, "somebody" for whom it is more important to swap light/dark colour schemes needs to design and implement it.