Because they just walk for no friggin reason. Outside they're too fast. Inside building they're way too slow. They need to find a balance for this. Zombies shouldn't run that fast. If you want to talk about realistic than muscular strength in corpses and infected people would result in slower, number movement, which means you are slower and weaker. Man have we got a badass over here or what? The zombies are way too fast as it is and see you miles away for someone who likes to play stealthy. If you can do that, which I doubt you can you awesome internet badass, good for you. Doesn't mean other people can. I sure can't. I take ages to loot a town because I have to prone and crouch all the time.