What I'd expect is two-fold - the heavy slide absorbs recoil energy that would otherwise slam into the frame, your hand, whatever follows the path of least resistance. It's possible to make a gun so light you don't want to fire it. This might be the reason why.
The other reason is simple stress - the lightweight slide slamming home with no absorption could cause the thing to shatter or knife-edge its way off the mechanism. With no mass, it slams into a stop and shatters or flies and keeps going. Material strength doesn't overcome huge, massive stresses or repeated problems.
A thin, light slide if it blows is literally shrapnel in your hand.
The other reason is simple stress - the lightweight slide slamming home with no absorption could cause the thing to shatter or knife-edge its way off the mechanism. With no mass, it slams into a stop and shatters or flies and keeps going. Material strength doesn't overcome huge, massive stresses or repeated problems.
A thin, light slide if it blows is literally shrapnel in your hand.
