Because the majority of the changes these agents deal with are likely simple and can be done following a simple web app that was probably available to the customer calling in. From the airline's perspective, the particular agent is likely still meeting productivity goals and doing most of their work correctly. They can hire people off the street with no specialized education, who may or may not have finished high school, to process the majority of their changes and it is cheaper for them to do so. Occasionally they will need to do something more advanced (but still relatively simple) such as your situation. Ideally they should be trained appropriately to deal with situations like yours, but you can only teach so much to some of their employees, that is especially "low-yield" in the grand scheme of things. If it is really an issue the customer wants resolved, then it will usually get taken care of due to HUCA.
Is it annoying and inconvenient that you don't have the same level of service from every agent? Yup.
Is it going to change? Nope, because to ensure high quality agents for every interaction would require paying them more to acquire & retain talent which is not necessary for the majority of customer interactions.