They certainly used to. Why do you say we don’t recycle glass? It’s basically the same process as making new glass, you just add broken up glass in with the silica sand.
You don't HAVE to, but you aren't going to be making clear glass again if you mix it with other colored glasses. Im not an glass expert though and it could be more of a cost consideration in not having to chemically separate out colored elements from glass when you can much more easily just get clean sand again for significantly cheaper.
