I agree we need to give accessibility more priority and we likely will once the discrimination lawsuits start rolling in. But this isn't an accessibility issue. Nowhere in WCAG 2 will you find anything that explicitly outlines both color and background color need to defined in stylesheets.

This is for users who prefer dark themes who are overriding styles of websites incorrectly. It is a failing of the user's browser/OS for not explicitly overriding both background and color at the same time or neither. If every developer had to add in a color for the edge case that someone is overriding styles globally, then we'd be bloating stylesheets with extra unnecessary bytes.