Meta gets a lot of flak for privacy, but at the same time, they end to end encrypt the majority of communication happening globally (Whatsapp+Messenger), at cost to the company, with no obligation to do so.
If by "cost" you mean Meta being in the business of siphoning user behaviour, Meta controls the E in E2E a.k.a the apps, so it's a matter of trusting them to not do covert on-device analysis + result exfiltration.
Many cybersecurity engineers passionate about this stuff have worked for Meta. They, too, would have blown the whistle at some point.
Not saying they do it, again it's about trust in the context of:
- Meta (the company, not the employees) having a bad track record
- Meta's business model being what it is (building profiles and selling tooling around that), creating tension with privacy matters