It is a very simple reason. They don't want to support multiple devices login as the same time to the same account. The reason for this I think it is because of

It is a very simple reason. They don't want to support multiple devices login as the same time to the same account. The reason for this I think it is because of their architecture constraints: - Currently, they are storing all the messages on the phone. So once the message is received by the phone, it will be removed from Whatsapp server. This can save the storage and processing cost on the server. If you want to support multiple clients, you need to save those messages on the server for a period of time (e.g. a few weeks). - If you allow two clients online at the same time, you need to support "carbon copy" meaning you need to send the messages to all the clients and keep the status of the messages in sync. This will take a lot of processing on the server side. - When a client offline for a long time goes online, you also need to sync from the server all the messages that were sent during the client is offline. So their architecture was built in such a way that the processing power on the server side is limited to the minimum. I believe that they may change its strategy in the future. However, it will take time.