Because its basically an expensive and pointless thing to do.
You might gain a Very, VERY minimal amount of protection from air-borne dust while changing
lenses, but what about the dust that is already on the back of the lens?...
Secondly any dust that lands on this shutter arrangement whilst the lens is off would still end up inside the camera body...
Also to make something like this would take up a lot of room internally, as well as being hard to design. And of course it is "Something else to go wrong"
The old Hasselblad V series camera bodies do infact have such a protective mechanical curtain, BUT this is on the BACK of the body, so to stop dust getting in when changing the back (it doesn't really work)
ETA: Damn, beaten to it by 5 seconds @matt Grum!