All the airline examples you list operate to Scotland from their hubs. That is how the airline industry works in the vast majority of cases.
The fact that CO fly EWR-EDI is entirely different to BA flying that same route. CO will fill a huge amount of their plane with people connecting onto the flight from all over the US, the same with all the other US airlines that fly into the UK regions.
BA act in exactly the same way, feeding people down to their hub in London, and then flying them directly to their destination. If they operated a direct flight from Scotland to the US then they would be competing with their operations out of Heathrow. This is frankly stupid unless you have a very very strong independent business case, as is seen with the MAN-JFK flight.
The hub system is the mainstay of aviation, and despite cyclical trends detracting from it (poor infrastructure in places like LHR currently, or the advent of new aircraft like the 787) this method will prevail for many many years.