iOS treats notifications as list items — similar to your emails where you have swipe actions.

Even if iOS notifications are presented in a list — I don’t see a reason why there couldn’t be a “clear all” action. There is a “clear” action and you can “clear all” with a 3D-touch. My guess is that many iOS users don’t even know you can 3D-touch ‘clear’ and clear all the notifications.

As for clearing a notification with a single swipe — I don’t see a reason why this wouldn’t be possible. For example, in Google Inbox you can configure what a swipe does: delete or archive.

My guess is that few people actually use the Notification Center — so it’s not high priority for them.