You can tell yourself that, but you've never checked out the PD's personal collection. The difference is you're programming for an audience of one, and the PD programs to an audience of many. And because of that, it's fine-tuned to appeal to many, not just the few music lovers. Some stations run "the director's cut" on their web site, and that's a lot deeper. But you can't play deep cuts and still attract the size of audience you need to stay employed.
Yes - I know that. I was attempting to be humorous. I'm sure there are rock radio PDs that go home and listen to classical music and have vast collections of opera on their MP3s.