Letterman hasn't been funny for maybe, 75 years, Conan can be funny but basically all the kids morning tv style skits are almost exclusively terrible(Conan + Matt Damon pretending to be in love thing was good). Interviews, sometimes decent Letterman says nothing interesting, asks nothing interesting. Because of all the filler crap on the shows the interviews normally amount to 2 mins of promo's for the latest film/show/album they are doing and nothing remotely interesting.
It's nothing more than a platform to promote crap and questioning rarely moves outside those area's.