Why is the white background really gray?

Because the metering standard of digital SLR is 18% gray scale. The result of measuring any object with a digital camera is gray and needs human intervention.

Portraits are normal, because the skin color of Asians is just close to this 18% gray level. It's not normal to take pictures of white people or black people.

It is often said that "white plus black minus" is the truth. Exposure compensation should be increased when shooting white objects, and decreased when shooting black objects.

Here is an article I hope you can read carefully, which will be very helpful to you! !