Why cant Sony just bring out a camera with a mechanism which covers the sensor when you remove the lens?

That would be extremely simple to engineer and i'm surprised that it hasn't been included in any of their mirrorless designs.
Leaving the sensor open to dust like that just seems like they are being lazy. It would need to lock open when you are cleaning the sensor, but all Mirror cameras already have that function. This is one of the chief reasons why I have never bought one - that and the fact that optical viewfinders will always be superior.