Because if it's end to end encrypted each device would either need the same private key to decode the messages or each message is encrypted with each devices public key.
This would require some kind of key exchange i.e. scanning bar codes.
There are probably a million ways to deal with it.
I haven't looked into it at all, but one way I just thought of right now is to have your own devices p2p the message among themselves with original sender's information after the one used most recently receives it.
I've noticed that a new device added to the account will not see the history. It only sees those messages that are received or sent after it has been added to the account.
This would require some kind of key exchange i.e. scanning bar codes.