The first reason has been explained already: They are afraid of (online) credit card and identify theft.

The second reason, why a lot of people (especially from Germany) will not be asked to provide a scan, is that they have a credit history in Germany. It is really common to check your credit history first. The german credit agency they are using probably has no information about you, that is why Hetzner wants to make sure that you are not trying to defraud them.

It can be really tough to buy stuff online in Germany if the local credit agencies don't have information about your credit history.