SLR cameras rely on a mirror flipping up to let light on to the actual sensor. Because the movement is mechanical it only has a finite number of clicks, even if that number is massive. I don't know many people who've had shutter failure.
It's also a direct reflection of how much use the camera has had. 400 acuations... it's sat in a box after the first month, 30,000 actuations... your buying my old camera hahaha
Think of them as km's on a car.