Distributed systems with distributed, synchronized state is a hard problem. Whatsapp handles 27 billion messages per day. That's almost 50x Twitter's volume. The former behaves like a switch with pretty end-points (the smartphone app), whereas the latter is a massive distributed system storing enormous amounts of state.

If Whatsapp allowed you to log-in from multiple places but didn't synchronize your state for you (i.e., see conversation history that you had on your phone after you've moved to your computer) it would be a pretty unpleasant experience. To accomplish that, the switch-like back end wouldn't be sufficient.