The MAC address, by TCP/IP standards, is never communicated outside of the local-area network to which it pertains — routers beyond that LAN don’t even get the information you’re trying to record.
There are many other ways to try and identify unique visitors, including matching the user-agent’s details in addition to the IP, serving cookies as part of your response, etc… it is, after all, a core functionality in the field of “web analytics”.
MAC addresses are simply not part of the gamut of techniques that it makes sense to utilize for it!