EU transit costs and peering agreements are much more relaxed and cheaper than in US
Europe is also a lot smaller network wise. Hetzner only have to get their traffic to Frankfurt to get connected to practically the whole of Europe. For the US, Ashburn N.Virginia is good but it's still only a single coast.
They are definitely paying under 2c/TB for traffic though.
Routers, optics & interconnects aren't free. $0.01/GB is very reasonable.