I think a lot of people are reluctant to go to the vet because the bird cost relatively little compared to the cost of medicine and an appointment.
My parents bought a parakeet a few months ago, and she fell ill within a day or so. They took her to the vet (fortunately, we have an avian specialist in the area), and he kindly did not charge anything to look at her, but the medicine to treat her was $80 (four times what the bird cost).
I have called the vet with questions about my birds before, and they usually can give you sound advice over the phone without charging money.
I think the cost really is a huge factor when people consider taking their birds to the vet. Unless it is an African Grey or something, people are going to figure that it is less of a hassle just to get a new bird.
Sometimes, however, the vet just isn't open, and in most areas there aren't 24 hour animal hospitals open. One of our birds died a few years ago during the night. We knew something was wrong, but we weren't sure what to do about it, and no place was open to take him.
I'm sure a lot of people are just lazy, however, and don't feel like taking the time to take their bird to the vet. They might also be children, and unable to take their bird somewhere to get help.
It is a difficult question to answer, because there are many scenarios unique to each person's situation, and while it is easy to blame people for being cheap or lazy, it is not always so simple.