Before the dark mode fad took off, people used to just do UI design and make interfaces that were just that - interfaces. They weren't attempting to be obsessively light or dark, so we got a wide variety of things in app UIs and the UIs didn't shy away from borderlines, shadows, color, darker elements in an otherwise not very dark UI, etc.
Then dark mode happened, and everyone wanted to design dark mode things. Dark mode makes small contrasts more legible (IME) and makes colors really, really pop. So you can easily design very flat UIs without anything in the way of borders, and small bits of color are really dramatic and eye catching.
Not-dark colorscapes feel really different - small contrast differences aren't as salient, you can use dark elements in the UI without disrupting it much. Borderlines and drop shadows look great, really high contrast for text just feels nice. Color is a much more balanced part of the whole, so you can use it much more liberally and it'll just feel nice and balanced.
But those are not the UIs we get today. Humans love to categorize things, and they like pairs of opposites a lot. If we have a specific dark theme, well, what do we call the other one? Light theme, of course. And in a disastrous display of nominative determinism, that's exactly what we've gotten afterwards: Designers have intentionally started making UIs that are actively light, and those are not designed specifically - most are just slapping white paint on the existing flat dark theme, or otherwise obey those same design sensibilities with light grays on white and minimal use of color.
Why would anyone want to use light themes if they constantly get designed wrong? The proper contrast people should be working with isn't light and dark theme or white and black theme, but colorful and intentionally dark.