Because they don't do crypto properly. Some friends of mine broke Snapchat quite badly a few years ago (one of them goes through HN, so he might read this). The tl;dr is that you shouldn't trust any aspect of your security to proprietary services. Especially ones that don't do crypto properly.
An idea that a couple of friends (can't remember if cyphar was one of them) and I came up with: piggyback some actual crypto on top of Snapchat, using steganography to initially transmit public keys. Would be interesting to see this happen.
I think I was there. Can't remember though. The issue is that Snapchat has gotten more stringent about image formats and things (remember when bad crypto could cause the app to crash?).
