you can do it ethically I'd imagine, but most people don't
Yes, but that's mostly because of the air travel. And most people who can afford to jet around the world will never be persuaded to choose otherwise.
Why is air travel so unethical ?
it's very bad for the environment. If you care about the environment and regularly fly, you're an asshole. Doubly so if you complain about other people not caring enough as you do about the environment or argue that plastic straw (or bags or anything) bans are a good idea.