Do you realize how many billions that would cost? An extra fifteen aircraft at six airports (and I am not counting GUM) = 75 airframes, figure $100 million each (and that may well be low) = $7.5 billion.

And it is not just the cost to own the aircraft, each aircraft requires expensive ongoing maintenance, aircraft parking fees, and each aircraft would require crews (and crews are generally not certified across many fleet types). OMG. Even if one reduced it to EWR & SFO and only two types at each place. Do you want ticket prices to go up astronomically?

Now, it might make sense for UA, DL, & AA/US to collectively contract with a Canadian airline to keep a spare 757 or two at YQX or YYR to be made available for wet lease on short-notice on a first come, first serve basis. But that would require some anti-competitive talks.