Because as a user, I want to change the light/dark of your site, not every set, and not my OS. If you don't have a toggle, you are making assumptions that aren't accurate.

I am assuming that if the user selected a specific brightness mode, they want sites they visit to respect that theme. Call me crazy but this seems like common sense.

I know some web developers think that that’s true, but looking at the average people I know, they tend to want different settings depending on the site. People don’t generally want computer-wide settings for darkmode.

This is true, also for people immersed in this world. Sometimes the dark mode of a site is ass, and it's better to set a preference for that site to use light mode to make it more usable.