Airlines also sell "points" for cash and you can get them for essentially the same cost as the credit card companies do. They're just using the 2% cash back to buy them for you, so even if you want flights, getting a 3%+ discount and using the money to buy points puts you ahead. It also lets you time your purchase (there are sometimes temporary "deals" on points), collect interest on the cash until you exchange it for something, and not lock yourself in to getting only airline miles instead of any of the other things you can buy with money until you decide that instead of having it imposed you at the point you use that card.
That's not really true on a practical level. For the most part, you can't just buy airline points at the one to two cent price that you effectively get them for in credit card transactions or the even lower price that the credit card companies themselves are likely paying.