Why does two different interfaces on the same machine result in two different MAC addresses?

Solution 1:

You are right that one device has one MAC address.

Your wireless card is considered one device.

Your Ethernet card is considered one device.

Every Network Interface Card (NIC) is one device and has its own MAC address.

Solution 2:

Each network interface, whether wired or wireless, is a unique device and as such will have a unique MAC address.

https://en.wikipedia.org/wiki/MAC_address

Solution 3:

A MAC address is an absolute necessity for communication on the data link layer of a network segment (basically every network wired/wireless). It is a possibility to have the same MAC address on two devices but no operating system or device is created like this, and under normal conditions will cause nothing but errors, as it will compromise the overall logic of Ethernet and network traffic will have no idea where to go.

I believe the overall effect of a duplicate MAC is to put the two devices in contention with one another, and to black hole any traffic to one of the devices. However it maybe dependent on the switch/router/topology used and you "may" not experience these issues, but its is definitely not a good idea and has few tangible benefits if any.

Its bit like putting same two numbers on a street and expecting the postman to deliver correctly.