What is wrong with the flight booking UX?

Seriously, why is it still such a pain?

Booking a flight online isn’t born yesterday. The first airline ticket sold over the web was in 1995, by a Palo Alto-based company called Internet Travel Network. One year later, Microsoft launched its Online Travel Agency (OTA), Expedia. Today, it is the #2 largest OTA in terms of revenue, right behind the Booking.com group.