Tor works in such a way that the data is encrypted by the three nodes. I.e., the entry node, relay and the exit node. But why is it not encrypted from the exit node to the destination server? Is this for speed? Or is it because it's not necessary as tracing back is already impossible?
Encryption of traffic exiting Tor and going to the destination server is based on whether the destination server supports encryption, and whether the destination server was addressed on an encrypted port - just as it would be had the traffic not gone through Tor.
Tor cannot magically do something the destination server doesn't support, or that the client did not request to do.