As far as I understand the workings of SSL - the client generates the pre-master secret and sends it to the server encrypted using the server's public key. Now, both parties perform some steps to generate the master secret and the session key. This session key is used for symmetric encryption from now on (e.g. communication in HTTPS).
My question is, given that we are sending the public key of the server to the client - and it is being authenticated and verified as well by using digital signatures of the Certificate Authorities - why don't we just use this public key for encryption?
Is it because asymmetric encryption is less efficient? Is this a trade-off between a one time less efficient operation (sending of pre-master secret and generating master secret) during handshake rather than multiple time asymmetric encryption? Any idea?
Clarification: My question doesn't suggest using asymmetric encryption for session keys instead i was asking about no session keys altogether, i.e. just use the public key sent to the client for encryption. See my answer to get what my question was.