Notifications here are for telling you how other people are interacting with you—they are expressly not for telling you how other people interact with each other.
Hence why interactions on reshares with content do not notify you, and why you can't see the reshares on someone else's post: those are interactions with the reshare author or with another post, not with you.
Regardless of whether you think that's a good goal or not I hope it at least describes the behavior in a helpful way.
