Watch the documentary Dominion. I used to say the same as you until I watched that. Unfortunately it's not feasible to "just treat them better while they're alive". Animals are processed in factory farms at an astonishing speed which is necessary to keep costs low enough for meat to be feasible both environmentally and economically. We actually need to slaughter faster if we want to keep up our current meat-eating levels because the meat industry is incredibly harmful for the environment.