Well a couple of reasons I can think of is firstly that Pippa does stuff in areas where she gets very occasionally papped - considering she has three kids and lives what seems like quite an active life she’s probably not papped the majority of the time.
I don’t seem to see any pap shots of the other Middletons or Markles on my feed. Maybe one of James just after he had his baby.
Rose lives on a private estate and I’m sure doesn’t do her own grocery shopping so she travels by car to other private places etc how would anyone ‘pap’ her easily.
We’ve seen her a couple of time when with Kate in recent years. So she’s not being hidden away by the palace.