They've answered this before. Their reason is that it prevents carrying chats over to multiple devices.
Discussion
Yes but doesn't encrypted backup and restore solve this? Anyway, hopefully they prioritise privacy over that...
No I'm talking about using the same account on multiple phones at the same time. If you go into a "secret" chat (encrypted) it only shows up on one device. They're not doing it like WhatsApp yet where it relays the messages.