> You can reverse engineer those things and analyze your network traffic.
I frankly don't think people realize how much obfuscation of both app code and network traffic goes on under the hood. "analyzing network traffic" isn't a sustainable option when things are encrypted and behind dozens of layers of protobuf, websockets and other fancy protocols, and get updated and change around all the time. Far from everything is introspectable http, javascript and json these days, and that applies espeically to big apps like these. It's not hard to send privacy-sensitive data along with "legitimate" data like analytics at unexpected times and evade scrutiny.
Yes there's people that dedicate themselves to reverse engineering apps like this, but they're few and far between, and most of them focus on either the easy fish, or security vulns. Considering nobody's building public documentation on the protocols of these apps I'll have to assume it's hard enough and changes often enough to be worth the time of people without special monetary interests.
I agree with the rest of your assessment, there's way less "obviously malicious" ways to exfiltrate data about users than literally uploading users' pictures, since for example whatsapp stored unencrypted backups on google drive until very recently, among other things. I'm just trying to shed a light on the fact that apps like this have a lot of ways to accomplish this without raising too many eyebrows.