How did they make these tapes sound so incredible? I'm getting more fidelity out of these tapes than just about any cassette I've ever listened to - even new in

How did they make these tapes sound so incredible? I'm getting more fidelity out of these tapes than just about any cassette I've ever listened to - even new in the shrink wrap. Amazing. This is like all of the great shows I couldn't go to in my childhood. Incredible.