It is correct that HTTPS and other protocols generate temporary keys for each connection but there's another much older reason why this is done, particularly for TLS.
Asymmetric encryption algorithms are, in general, orders of magnitude slower at encryption than AES. With these protocols the initial secure connection is done using asymmetric cryptography. It makes sense to use the established secure channel to exchange another set of keys and switch over to AES or another symmetric algorithm immediately.
Nowadays the two ends negotiate a key exchange to allow things like perfect forward secrecy as well, so this is becoming a historical footnote to an extent.
Asymmetric encryption algorithms are, in general, orders of magnitude slower at encryption than AES. With these protocols the initial secure connection is done using asymmetric cryptography. It makes sense to use the established secure channel to exchange another set of keys and switch over to AES or another symmetric algorithm immediately.
Nowadays the two ends negotiate a key exchange to allow things like perfect forward secrecy as well, so this is becoming a historical footnote to an extent.