I think what docker_up is saying is that if all materials were recyclable there wouldn't be "other junk" to mix in. I actually agree strongly with this, although I'm not sure how it could be practically implemented.
Because it contaminates everything else, making it much harder to recycle the other items.
Because there's no reason to recycle it, the planet is made of it, we're not going to run out, and it's impossible to waste it.
Do you know what they do with recycled glass right now? They crush it, and use it as a landfill cover (instead of sand). There's little point in spending all that effort to do that.
Almost all plastic is recyclable already - but not when it's mixed with other junk.
What they should do is stop recycling glass, and recycle paper separately. That would solve the problem much easier.