I'm not sure what your basis for assuming there's another A319 just hanging out. Aircraft utilization is very high. If it made sense to switch you to another aircraft, they would. But it's not like RDU will close before you get there, or any passenger on your plane has a connecting flight out of RDU they might miss. It would be more convenient for you, but airlines make decisions based on their entire operations, not you.

If airlines kept lots of extra planes around as backup (they'd need one of each type of aircraft at each airport they fly out of, since crew is qualified for specific aircraft and each plane seats a different number of passengers), that would be extremely expensive. That cost would be passed on to customers. 8 of 10 passengers would fly on the cheaper airline on the same route who doesn't have backup planes, as history has made clear, customers really only care about price.