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

There is no "router of the server". Packets may reach the server from many routers.

There is no "router of the client". Packets may be sent from the client to many routers.

The only devices which can see a MAC address are those on the same LAN - possibly only those on the same cable segment.

BTW, the server cannot get the client IP address, either, if the client is on the other side of a proxy server.