There are practical problems. Does no internet also mean no network connection? Getting two devices to talk to each other without a network is not straightforward. Bluetooth can do it but has range limitations. Wifi can do it to, but there you have to get into very hardware-specific wizardry to move packets between devices without a network router. The target devices, cellphones, are designed to provide network access, to login to a backbone of other devices, not to generate and manage the network themselves.

Then there is the issue of encryption. Verifying keys is tricky without a common network. How does one revoke or renew a key? It is possible but complicated. Not something that a messaging app would normally attempt.