I think the answer is due to money and pride. My guess is that the issue is billing and international relations. Code shares aren’t (often) between airlines tha

I think the answer is due to money and pride. My guess is that the issue is billing and international relations. Code shares aren’t (often) between airlines that are from the same country. So, if you have a requirement to fly a particular flag carrier, you have greater flexibility. For example if the ticket is funded by the US gov’t, you can still take an Air France flight, but get billed by Delta. Or, for frequent fliers (whom airlines love to target with rewards), you can have miles accumulate with United even if you are taking an Air Canada flight. But I would think that this change could be made on the backend much easier than trying to add another number to the flight IDs.