I wonder how long the design engineers struggled with, "do we or don't we".
Personally when I change lenses the one that will be put on is ready to go, lens rear cap off, and the amount of time the camera's inners are exposed is only a few seconds. Even then dust will find a way in and eventually find its way to the sensor
I suspect that shutters today are so lightweight and delicate that they offer very little real physical protection. Of course, they do provide some minimal protection against contamination, but maybe with the shorter distances involve, they have become targets for damage themselves.