In that case, can the airline decide to simply not run the flight and keep everyone's money? It's a contract because both sides have accepted some number of terms. If it weren't you would forego all sorts of protections. When you (reasonably) want those back... guess what? Disputes are decided on the terms of the (sales) contract. How else would it work?
Maybe if a ticket actually held some sort guarantee for getting a seat on an airplane things would be different...
No they don't. They overbook to maximize profits, yes. It sucks and is completely anti-consumer, yes. However, they do not simply send you packing. They re-book you. You also agree to it when you buy the ticket. Again, yes, it's buried in the contract and since every airline does it you really have no choice.
It's far from perfect/fair, but that has nothing to do with what I responded to. If "everything doesn't have to be a contract" then you have no legal recourse. Ever. Sales need to be contracts in the eyes of the law, but that doesn't mean everything is perfect.