Carriers live and die by turnaround time to keep their jets making money (or at least, not losing money as quickly...) A significant part of SWA's competitive advantage is their industry leading turnaround time.
Having every carrier have to park a standby jet (even just one), flight crew and full cabin crew at each of their airports is going to flow pretty quickly (and stoutly) to the ticket price. (There are duty limits for flight and cabin crews, and being on active-standby, such as you'd need to be in order to effectively reduce delays, counts against those limits.)
Consumers want cheap tickets. Airlines are providing that, sometimes at the hidden expense to those consumers of delays and cancellations.
I agree with everything you said about the cost. However, large airlines do keep hot spares at a few locations. Low turnaround time is great, but sometimes something major breaks and you need to be able to pick up the slack. There are even companies who provide "generic" airplanes for this purpose.
PS: The incentives are somewhat skewed. Customers who have already paid for a flight still need to get where they are going so they accept delays.
Having every carrier have to park a standby jet (even just one), flight crew and full cabin crew at each of their airports is going to flow pretty quickly (and stoutly) to the ticket price. (There are duty limits for flight and cabin crews, and being on active-standby, such as you'd need to be in order to effectively reduce delays, counts against those limits.)
Consumers want cheap tickets. Airlines are providing that, sometimes at the hidden expense to those consumers of delays and cancellations.