Actually, you have to assume that any encryption is a temporary measure; it can and will be broken in the future. With that in mind, if something is worth encrypting, it's possible you don't want the service to store it after it's useful.

Of course, whatsapp could register devices which are thrown away after x days without delivery then store the messages until delivered to all end devices but that's likely a core infrastructure update which would require them to store more data, not the same as just writing another client.

Still, what they've created as a web client is a little batty; typically I'd want to use those when I can't find my phone.

I assume investors of telegram are Russian big dudes, and they use it to control people. Thats why they store it on their server.

I know, this is a wild guess. Id rather use whatsapp.