The 'oldness' of a lot of the music on some active rock playlists makes the music sound more stale.
Once again, this is a music problem, not a radio problem. Radio is simply reacting to where the core audience is for the music. Active rock has very few current consensus bands. By that I mean there are very few artists they can play that everyone loves. A lot of those current acts are making music that is very derivative from older rock. They're not advancing the music much further from where it was 25 years ago. If you're a kid hearing classic music like Led Zeppelin or The Who for the first time, it's not stale to you. Especially if current acts are basically doing the same thing.