That's because politics in the US isn't pluralistic but a static duopoly that's treated like a team sport by a lot of voters. In Germany the opposing player to the far right is the Greens for example because the two parties are both benefiting from the cosmopolitism vs communitarism cleavage that drives a lot of political debate. Voters will hardly switch between these two parties specifically. But if you are in a de-facto two party system where the opposing party is at the opposite end of each cleavage then you'll barely have any voter movement. It's entrenched this way for decades

Most democracies are pluralistic with more parties which is why the Germany anecdote makes more sense than the US one