Its that the old stuff is still around...because something happened along the way, and nothing really great or unifying in music happened much after my younger years, and the old stuff is still strong enough to keep a following. It hasn't been supplanted yet.
I think you still have your rose colored glasses on.
That's the stuff I listen to too, but remember, you are listening to the best of the 70s.. Just like people are "still" listening to the best of the 50s and 60s..
There's still lots of popular (literally) music. I don't listen to it, but I know it's out there. Some of it may be the "they're still listening to it" in 40 years. (Pink maybe? Adele, though you'll probably just mention that she's in the style of older music.)