Why might ghettos have been an entirely eastern phenomenon, not built in germany or other nazicontrolled areas?

In Nazi, Germany they were enforced ghettos. Jewish people were forced to move into them to be kept separate from the general population and make them an easier target since they were already isolated from others. In the US, ghettos emerged because when people emigrated to the US they had a tendency to settle down among their own ethnicity, so the ghettos sprang up from that.