Why don't airlines provide a good free API for flights and reservations? I would think they would want developers to help make accessing their offerings and buying them easier.
Since finding and booking flights is actually trivially easy for the consumer already, it's actually in the interests of airlines (as well as the middlemen) to be cautious about who gets access to which API function, especially when it comes to actually selling tickets.
At the extreme end of the scale, some low cost carriers can only be booked on their website, because they make more in upsell from the booking flow than they could make with extra bookings from other channels
But other than that, I assume there's a lot of money in partnerships with sites like Kayak and Priceline. But I'm not even sure which direction that money flows.
Also they don't care about giving coach passengers a pleasant experience. They'd remove the seats altogether if they could.
They have their own ticket shop online. So they prefer if people buy from there.