Short answer: The perpetual plastic project recycles plastic to print. This is very, very difficult though. Making fillament at all is already difficult, recycling it adds even more complexity.
Long answer; There is a lot that you need to do to get it right. The main problemn with PLA for instance is that it degrades at the printing temperatures. This means that recycling it will ruin your material to some extent. ANother big problem is how to correctly mix the materials. As said; You want a perfect mix of the material, but with more dye added to prevent the ugly color. But leaving it longer in your extruder will degrade the material. This is usualy the reason that you don't use 100% recycled material, but addd some new material to improve the quality to acceptable levels.
So now you have the material right, you also want fillament that has a good shape & consistency, which is an art onto itself.