There sure are many technical reasons to not have e2ee by default. In Signal, at least as recently as in August, you may still get messages from the group you

There sure are many technical reasons to not have e2ee by default. In Signal, at least as recently as in August, you may still get messages from the group you just quit if a member send a message to the group before the group metadata is reconciled. This happens because every group message is an e2ee message to every other group member. If you have signed in from two devices, disappearing messages that have disappeared on one device will still show up on another. This happens because the server cannot store or sync the states of two sessions participating in the same chat. Signal is full of little distributed system data consistency bugs that you just wouldn't expect in traditional client-server model application.