Sure but United (or whoever) chooses to sell them tickets and create those experiences.
Delta has a product called Delta Vacations, run by Expedia. All of these things are businesses decisions they decide to embark on with 3rd parties. They promote the integrations. They don’t have to do these things. I don’t mind if they do, of course, but I have 0 sympathy for them if someone has a bad experience on a 3rd party site selling their product.
If I buy something from Amazon it’s no different.