Several reasons, Icemaiden. Firstly, I think 2-3 y.o hens would be too tough to eat, and dual purpose hens would be likely to lay less than hybrids and to go broody which is a nuisance. Secondly, dual purpose breeds can make sense if you hatch your own and grow on the otherwise unwanted cockerels to about 16 weeks and then cull them for the table, but I haven't either be room or the desire to do this.
Lastly, although I do now cull my own birds I really hate doing it, and even if I knew how to pluck, eviscerate and prepare the bodies for the table, I couldn't eat them because they do all have names, I do treat them as individuals, and I do feel sorry when they go. I give them 5* treatment all their lives in return for the eggs, and for the interest and life they provide in the garden, but I don't get attached to them to the extent that I lose sight of the fact that they are there for a purpose, and I don't want a lot of old birds laying soft eggs now and then, slowly degenerating into prolapse and peritonitis, whilst I'm buying eggs as well as feeding pensioners.
But I do understand that many people keep their individual oldies well on into old age and cherish them to the end, and that's fine. I used to do that, and I used to take the oldies to the vet to be PTS, but after many years of chickens I've gradually changed my view of how I manage my very small flock (3-5 birds max.) I do think that all keepers need to know how to dispatch a bird quickly and efficiently in an emergency, and having learned this, it seems best to keep in practice now and then, and it's a lot cheaper than the vet as well.