Back to your original question, "NY-LA, why is flying cheaper than Greyhound?"

It's incorrect to assume that just because something is more in demand, it must be more expensive. Prices also have to do with the cost of the item and the competitive landscape they are in.

For the bus, you need to buy multiple products and combine them. Thus you are paying for prices that represent demand on these other items, since there probably is very little demand for a single NY-LA product.

Cost-wise, I'm not sure. Certainly airlines have a huge fixed-cost in the plane, pilots are more expensive than drivers, and jet fuel more expensive and less efficient per mile than cars. But, remember that a plane can hold many more people, and certainly not all passengers are going to be paying the rock bottom price!

So in sum I'd say it's complex and there are many factors to explain it, but seems perfectly possible it could be cheaper or more expensive