Why is Yangchengtong limited to one person by bus and subway?

One person, one card, one machine, one card, you have to swipe your card when you go in and out by subway. You can't swipe your card into the station for the second time before you leave the station. You can only take the subway alone. However, taking a bus is a bit different, there is only one direction. After swiping one card, you can swipe the other three cards, which means you can swipe two people. It's ok occasionally, but it's often not.