I don't want to say that Debbie Wasserman Schultz has no political power at all but her power is miniscule compared to the head of a German political party.

That's because of differences in the electoral system. In the German system people vote for party lists, which the party heads choose, in the US system people vote directly for politicians; furthermore, each party's candidate is decided by another election, called a primary, this leaves a lot less for party officials to do.