I do realize that hens will lay for several years before they stop. But by replacing them every year (in theory) we get to enjoy having chicks, we can get a good price for the year old layers, and the buyers get a good deal on healthy animals in the prime of life. Also, since I don't have any desire to slaughter chickens if we didn't sell them, then eventually we would have a flock of decrepit old girls that lay narry an egg, and we would be forced to watch them die of old age one by one - while we buy both chicken feed and eggs.
Eventually the back yard would be filled with tiny little crosses and it would just be too sad.
For my family livestock can be pets, but they are still livestock. I hope you don't think I'm a monster.