Why sensors do not cover the full exhibition circle?

Imagine having a round sensor covering the entire circle that your lens can exhibit. Your camera should be able to take pictures in any aspect ratio in any orientation. You will not have to rotate your camera for portraits; just set the 2: 3 format instead of 3: 2 …

Your collection of lenses is more expensive than your camera. the sensor of your camera only represents a portion of the cost of it. Why then do camera manufacturers not produce circular sensors (which would be only slightly more expensive) and not provide the aforementioned amenities?