Why do they need to have a special train for each airport? What's so hard about putting them on the regular subway system so people don't need to pay a high fa

Why do they need to have a special train for each airport? What's so hard about putting them on the regular subway system so people don't need to pay a high fare and waste a lot of time in transfers and walking between stations with a ton of luggage? Have you even been to Penn Station? It's not a short walk from the trains to the subway. That's probably why a lot of people prefer cars.