Why can't the server get the client MAC address, like the client IP?

Question

From what I know, all the MAC addresses that the packet gone through are in the packet. This is because that each packet that goes in a certain path, should also be returned

Answer 1

In IPv6, one might actually get the computer's MAC address from the IPv6 address, if encoded into the 64 bits of the host address.

See also How to avoid exposing my MAC address when using IPv6? on Super User.