I read that why do we need E2EE and can't rely only on HTTPS for sending messages through a messaging app. The reason which i understood is when sender sends the message to the server, the TLS connection is associated with the server. TLS terminates at the server and whoever controls the server has the ability to view the messages since they are not encrypted.But, In this process when we send a message to the server, we are firstly encrypting the message with sender's private key and then with server's public key.
My question is why can't we encrypt the message with sender's private key and then receiver's public key? In this way, even if it reaches server, it won't be able to view anything since it can only be decrypted using receiver's private key.
If this is possible, then why do we use methods like Diffie Hellman key exchange?