Core reason why? Their limited shelf life. I happen to own the original Doom series, and in a fit of nostalgia, I decided to reload it on my new computer. After a hell of a lot of tinkering, as well as using an open-source engine mod, I was able to get it running again. More often than not, I've ended up chucking my old games simply because it's not worth the effort to get them working again. Yes, I have DOSBox on my system, but seriously, what's the point?
Manufacturers aren't going to be marketing them, and to be honest, few people are going to go through the effort of trying to make it work on new systems. So complaining that they shouldn't move into the public domain long after their day in sun is done feels more like "my precioussss..." than any economic argument.