Maybe someone keeps them long enough to determine which are roosters and hens then get rid of the roosters as soon as they possibly can yet get something out of it. It costs a lot to feed a dual purpose breed rooster to get to a larger size so they process them as soon as they consider them big enough to get some meat out of it. That can include pullets too if they have more pullets than they need.
Age makes a huge difference in how they cook. If someone insists on frying them or grilling them, they may need to process them before they get too old.
I prefer to wait until later. There is a lot more meat on an older bird and I like the flavor of an older bird. I cook them slow and with moisture anyway so them being older is a plus for me, not a negative. Mine forage for some of their food so the cost to feed them is not unreasonable. I eat the roosters first and let the pullets lay a while before I decide which ones I want to keep for my permanent laying/breeding flock. I tend to process a fairly small amount, 6 or less, at a time so my freezer is not filled with meat in case of a power outage. I make broth from the parts we don’t eat and older birds make better broth.
We all have unique circumstances and different goals and criteria. You don’t have to do what anyone else does. You need to figure out what suits your purposes better and do that. It may take some experimentation.