A better question might be "who designed my network?" and the fact is that there's no home, given the routing and addressing protocols used to service HTTP requests, for my MAC address to be in there.

You seem to be suggesting that some monolithic entity has decided to quietly alter these protocols to include my MAC address somewhere, and I think that's ridiculous. I mean, that would have to be one hell of a conspiracy, given the number of people relying on things like the IP and TCP protocols, and inspecting network packets down to the actual bits. Did the Chinese, or the CIA, or whoever convince Wireshark to just hide the extra data, lie about packet sizes, etc.? What happens to the poor guy who tries to make some compatible device or program, and dutifully follows all the specs, only to find he's off by however many bytes because there's now secretly a MAC address in there?

It's not an idea that survives scrutiny. It's far more likely that LBJ killed JFK or that Bigfoot exists.