Landfills are engineered at some cost to retain nasty liquids and gases that are produced. Glass doesn't need any of that. Or, if you're going to put glass in

Landfills are engineered at some cost to retain nasty liquids and gases that are produced. Glass doesn't need any of that. Or, if you're going to put glass in a landfill, it could be cheaper to put it into a glass-only landfill that wouldn't need those protections. If you're going to burn the waste them removing glass first makes that easier.