That's not entirely true. I throw away a lot of cardboard packaging with a plastic window glued into it. Obviously this can't replace all plastic, but it can certainly replace some.
Plastics do a lot of things; no one material can replace them all. But this is certainly one meaningful niche of disposable plastics.
Curiously, a lot of tape is made from "cellophane", which is chemically similar to paper. The windows on envelopes are typically made from it. Not sure about cardboard boxes but you can make cellophane pretty strong.
Plastics do a lot of things; no one material can replace them all. But this is certainly one meaningful niche of disposable plastics.