The hotel chains can get together and offer a website showing lower prices to their rewards members.
In fact they did, called roomkey.com Hilton/ihg/marriott/hyatt/choice/wyndham options could all be searched simultaneously, and you get the lowest price and the hotels avoid paying commission to Expedia and booking.
But they shut it down. I wonder if it was because hotels are usually franchised, and the brands get a percentage of revenue as royalty, so they don’t care about commissions the hotel operators have to pay. So they decide to price discriminate via the travel agents.
It should be similarly trivial for the airlines and car rental companies to get together and offer the same website. Isn’t it better to give customers a 7% discount rather than give booking/Expedia 15%?
Obviously, reality is different so there must be a reason. I’m guessing the gains from price discrimination are more than the losses from commissions.