Because back in the days people used RGB without a spec (even though sRGB exists since 1996). RGB on its own doesn’t mean anything, so you get random shit on screen. Any variant of RGB (sRGB, Adobe RGB, no spec RGB) should only be used for presentation, not for composition. Composition should be done in a colour space which is luma aware.

That’s the issue with RGB colour space in general. That’s why when you’re working with anything graphics related (3D modelling, colour grading, etc) your first step is to switch to a different colour space.