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

It's source & destination IP address which remain. The idea of inter-network is basically based on hop-to-hop delivery. Intermediate routers/host understands only MAC addresses. To my knowledge routing tables should understand machine address.

If MAC addresses could remain throughout the journey of a packet; which means there was no need of Internet Protocol addresses to be invented. And all of the internet would have used only MAC addresses :-)

You can achieve what you want only if both the server & client reside on the same local network (both hosts are connected by some L1 media).

Having said that, your application looks something like a Web Server, which tells me that it need not be on the same network.

Still if you want the MAC address of the source host at the receiving host; think to sent it as a payload?

Addition:

Networks need not be of the same type around the world (viz. Ethernet, Frame relay etc). Network layer provides us with the flexibility of routing without distinction between underlying layers(Data Link Layer) or I should say underlying L1 technology. In short, IP will give us the inter-network and Data Link Layer(where MAC addresses come into picture) will take care of communication at micro level (i.e. local network). This is a fair reason why Mac and IP addresses co-exist! :-)