I don't think that's necessarily true as a general statement. There are also lots of knockoffs where they clearly aren't actually the exact same product because they're obviously slightly different in a way that shows it wasn't produced with the same dies/molds. I own several knockoffs like that (which I bought intentionally knowing they were a knockoff but the knockoff had good reviews and it wasn't worth spending extra for the original).
Occasionally there are even knockoffs that improve on the original product in some way, although usually they are just nearly as good at a much lower price.
Occasionally there are even knockoffs that improve on the original product in some way, although usually they are just nearly as good at a much lower price.