Because you’re mixing different light sources with different color temperatures. The ambient scene is likely lit by warm light, while the flash is closer to daylight. White balance can only neutralize one light color at a time, so the camera must choose which light to treat as “white.”

With flash enabled, auto white balance often assumes the main subject will be lit by the flash, so it biases WB toward flash/daylight. That can make the ambient-lit parts of the frame look yellow. If you instead apply the non-flash WB, the ambient may look better, but flash-lit areas can turn bluish.

So this is not just the camera being “dumb” — it’s a real limitation of white balance in mixed lighting. Cameras use simple rules of thumb; they do not interpret a scene the way human vision does.

If your camera has a flash WB mode, it may help for flash-lit subjects, but it still won’t make both flash and warm ambient light look neutral at the same time. To fix it, you usually choose which light should look neutral, or make the light sources match more closely.