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Mac addresses are used in networks, not in internetworks. So, why to not create a protocol, that will automatically assign to connected devices MAC address in that way, so they don't repeat in network? So, we could use less number of bits in MAC address or more devices can be created not worrying about address.
More generally, why uniqueness of MAC addresses are so important? We use them just in networks, so we shouldn't worry about their uniqueness, except in the same network.
You can create your own MAC address. It's called a Locally Administered Address. You indicate it by setting the the second-least-significant bit of the first octet of the MAC address to 1.
So, why to not create a protocol, that will automatically assign to connected devices MAC address in that way, so they don't repeat in network?
Consider how you would use your new protocol if every device on the network wasn't addressable (didn't have a MAC address). How would you distinguish one from the other?
So, we could use less number of bits in MAC address or more devices can be created not worrying about address.
There are dozens of ways how networks could be built. MAC addresses and the rest were developed at a particular time for particular reasons.
With hindsight, it's easy to look back and say, "I have a different idea that works better." Even if that were true, ask youself, is your idea so much better that it is worth replacing the hardware of every single Ethernet device in the world and updating the software on each of those devices? Does your new protocol improve things so much more that it is worth the millions (billions) of dollars and tens of thousands of hours of work to replace everything? Do you want to buy a new computer, an new phone, a new tablet, a new television, etc. just to use your new protocol?
There are many, many things in the world that are the way they are because that's what someone decided, and changing it years later for the sake of change is simply not worth it.
The problem with any multiple-access network (Ethernet, radio protocols and so on) is that even to assign an address at startup, you need be able to communicate with the device to tell it "here's your address".
Now, how do you tell THAT device the communication is intended for them? Well, you need some kind of unique identifier (think DHCP which relies on layer 2 MAC addresses to do this). So you have a bit of a chicken-and-egg problem. You could use a random identifier, but we all know "random" is a lot more difficult than it looks like if you want to avoid any collisions.
So nearly every protocol with multiple access has some kind of unique identifier, be it the MAC address (of various lengths), or some other. Check Ethernet, Wi-Fi, Bluetooth, Zigbee, LoRaWAN... I suppose cellular networks use the IMEI for this purpose.
Nowadays, Ethernet is not really multiple-access (there is no shared medium on switched full-duplex networks), so with managed switches one could imagine assigning addresses based on what port of what switch they are connected to, but this can quickly become quite complex (How do you assign switch identifiers? How many bits do you assign to the port number?), and with the move to the all-wireless world this is not an universal solution anyway.
Remember also that MAC addresses are used in many higher level constructs (ARP, DHCP, IPv6 addresses...), and that networks span multiple physical technologies (Ethernet and Wi-Fi), and that makes getting rid of MAC addresses quite difficult, even if it were practical or useful at layer 2 in some network technologies.
