Because every time I break in another dx roc to straight, I have a backup for that slot in my bag. I have my rocs sorted in the garage, I have a pile of new ones, 10 straight ones and a few beat ones so that if I ever lose one of the rocs in my bag I don't have to wait for one to be ready to replace it. I still disagree that they change constantly, my current straight roc has flown the same for close to a year now, I know exactly what it's going to do. Ditto for the beat one, at this point even a point blank tree hit doesn't change the way it flies in a noticeable way.
The other part of the answer to your serious question is that none of the premium plastic rocs fly like a dx roc. They get more distance, have better glide, and the grip is way better. I have yet to throw any premium plastic mid that could replace my really beat roc for shots that start on a hyzer, flip up to dead straight for 275-300' then gradually finish right. Those premium plastic discs either are too stable and want to fade, or don't have the HSS to get that long straight flight segment. Also, I'd need 3-4 different molds to cover the shots I currently use my roc setup for, right now every mid shot has the same grip and the same feel in my hand which in my personal experience makes my mid shots more consistent.
All of that is why I do it. You're right that you can be a fantastic player without throwing base plastic discs, and without cycling rocs. I just disagree that my strategy has been made obsolete by newer discs and plastics.