It's actually simpler than that. It's a lack of self confidence.

An agent many not know how to do something, and instead of admitting that and finding out, many will just make up an answer to get you off the phone. They may think that they'll look better by not admitting they don't know something (when in reality, we'd rather they just find out). Or they might be too ashamed to ask their supervisor for something, thinking they should know it already, so they make up an answer.

This is not true of just airline agents, but any agent, any cashier, any customer service person, anywhere. Think along the lines of a $7 an hour person in any front line position with inadequate training, anywhere.

Factor in that these people are often rated not by the quality of service they provide, but by the number of customers they handle. If your call would require a hold for a supervisor to research an answer, they could be better off telling you it can't be done, and having you buy it, since they get credit for processing another call.

People get BS'ed many times a day. It's a matter of managing that BS. Either hang up and call back, or escalate and make an example of them if you have time to do so.