Why Even Have A Shutter Mechanism Anymore?

I was just wondering why it's even necessary to build in a complex mechanical shutter mechanism into moderm DSLR's that's prone to eventual failure due do its many moving parts, when all you would have to do would be to "switch on" the sensor for the needed exposere interval? After all, isn't this all that's happening in video mode, not a shutter flapping away at so many frames per second.
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Jerry Brendle