A ton of them are recyclable in their pure state, but I get the impression that a lot of seemingly mundane plastic products these days are actually surprisingly complicated mixes/composite materials: I know something simple like a potato chip bag is at least a 3-layer composite structure, and you've got inks from the label, metal from the barrier layer, polypropylene from the film, and polyethylene from the adhesive layer.

On top of that, the polypropylene film itself is likely a multilayer structure of different copolymers and crystallinities, and it all turns a crappy little thing like a chip bag in to something damn hard to recycle.