Why is glass allowed onboard airplanes?

Box cutters and even the tiniest knives are forbidden on airplanes. But what’s to keep someone from bringing a piece of glass on board, shattering it into triangles, and winding a piece of cloth around half of it to use as a handle (a la Raven in Neal Stephenson’s book “Snow Crash”)? Surely that’s a bigger danger than fingernail clippers.