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.

It could otherwise be reused? A lot of glass containers come with a deposit and then are reused once returned. This happens with some glass milk containers (exactly why I dont know). If container shape and size became somewhat standardized this could work better. Glass can be reused while not shedding microplastic everywhere in the process.

Landfill space is limited, best only to throw out what cannot be repurposed from a physical space perspective.