please explain the shutter function

Ok, so I have one question, perhaps its been asked before but I can't find the answer. Why is it on Mu43 that the shutter is always open hence the sensor is exposed when you change lenses? Can the shutter not close like a DSLR when you turn the camera off, so when you switch lenses the sensor is protected from dust and whatnot? And when you turn the camera on, the shutter opens and then it functions like normal. Any explanation? I wonder. Thanks
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