Nitpicky, but he’s rounding the edges, not the corners. And yes, why are they so sharp? I seem to recall my wife having the plastic MacBook that came out circa 2006 and the edges on that thing were legitimately painful. I always marvel at how sharp the points are on the notch of the lid on my current MacBook. Very very pointy.
I have intrusive thoughts of trying to cut my finger over it, but so far the attempts were unsuccesful.