Background color is always render slightly brighter, why and how to fix? (Eevee)

I noticed even with a blank Blender project with nothing but a camera the background color is rendered brighter than the background color in the settings. Why and how to fix?

Try looking at Render Properties > Color Management. If you want them to match select Standard/sRGB. For the reason why you’ll need to explore the difference in the color management system. I use Standard for static images and some motion graphics. But Filmic for video.

What’s the difference between “Filmic” and “Standard” when they are both sRGB?

sRGB will easily blow out bright spots. Filmic tries to mimic film emulsion where adding more light makes it brighter, although in real emulsion this will when taken too far start to add noise. This happens by way of manipulating saturation at the higher end. If you want to use sRGB you have to apply some kind of local contrast adaption with some kind of post process tonemapping technique.

If looking at a film photo of the sun, you will have a bright sky with the sun even brighter. In an sRGB photo they will all be full white with sun blown out. This is exaggerated but you get the point. By manipulating saturation, filmic is able to show a deeper range of intensities. It’s a no brainer no effort “tone mapper” which simulates what analog film captures, but not so much what your eyes see.

It’s 2020 why have analog film mimicing method the default?

Because it gives instant tonemapping with no fuzz or adjustment. You can set to standard if you want and save the startup file. But then be prepared to spend time in post doing tonemapping - something I could never get the hang on in Blender.
DSLRs also shoot deeper than 8bit per channel, allowing a deeper raw image where you can do tonal adjustments in post, such as “highlight recovery” and level adjustments without causing stepping effects.