if you were Kayak, and you already had a backend in place, wouldn't you assign at least one designer/developer to play around with novel UIs?
What's to say the current players haven't already tried those UI changes and found them less usable by the general population?
I don't know for sure, but I do know that calling the current competitors lazy is the completely wrong view (and factually incorrect). First, they deal with an older, less "techie" audience, which means different UIs are immediately viewed as confusing rather than cool. Second, they have the traffic to run A/B tests on everything, which allows them to know what such changes will do to their bottom line without having to publicly deploy or announce them.
Full disclosure: I worked on TripAdvisor Flights and was in the same YC batch as Hipmunk, though there was no overlap between the two and I actively avoided mixing the two circles, so I don't actually know anything about how Hipmunk views TA and vice-versa.