IF don't fight it and if you respect semantics and progressive enhancement / beautiful degradation. You get so much by default: legible colors and font sizes, tabbed page navigation, alt texts, clickable labels, zoomable interface...
When you embrace the platform and its spirit, you get accessible results quite easily. But not a lot of people seem to embrace it. Many people seem to think "what to do for what I want to see" instead of "what to do for what I want to mean", and you lose if you do this. You need to get your meaning right and then decorate it carefully. You should not paint first. But that's probably not a web-specific issue.
What's more, building accessible things may make you gain time, by having a robust basis. It's not always a cost.
There are native UI frameworks where this is not nearly as big of an issue.
Too much freedom perhaps? Should you even be able to give a div an onclick handler?
What about inputs? Why do developers have to make custom inputs because the core ones are severely lacking? That's where accessibility goes wrong. Because core functionality is terrible and needs to be augmented with richer functionality (but terrible from an accessibility perspective).
There is something fundamental about this platform that does not naturally encourage good behavior. All you have to do is look at the evidence.
As an dumb little example, it's not even obvious that making a div a button is bad unless you know that it is. The platform doesn't guide you at all. It has an onClick handler the same as a button. Little attempt is made to differentiate the two from an API perspective.
I could give countless other examples, but I think I have demonstrated what I am getting at.
