Why is it fine for shotgun shells to be made of plastic but polymer casings for rifle cartridges are considered “unreliable” or “bad” ?

Question

I’ve never fired/handled guns but have some knowledge of guns and their basic mechanisms, and this thought just came across. If you could make cartridges out of polymer/plastic, and they are significantly lighter, wouldn’t you want to switch to them asap? yet there is no adoption. On the other hand shotgun shells have always been plastic but it’s not problematic in any way? why is this the case?

In: 24

Answer 1

I think it’s because shotgun shells are supposed to exppode and release the little pellets inside
Edit:fixed typo