The fact that nobody really wants a true p2p platform: you don't want to lose all your stuff if you lose the device. It is clear that federated services like email are superior for most users bar some real hardnuts mindfixed on privacy.
Why not provide encrypted backups to a personal cloud storage service? Even WhatsApp does it these days.
Ah, btw, how will a true p2p platform work on iOS? What entity will send you push notifications when there are messages for your device?
True P2P on iOS: No idea, I merely claimed that a lack of backup options is not the reason we don't have it.
In practice, nobody wants any of that. The vast majority of users are perfectly happy (and for the right reasons) with federated and centralised services like email or WhatsApp/etc.