Germany not only had spies, there were multiple (albeit failed/foiled) sabotage attempts by Germany on US soil.

Part of the issue the US had is the very large (significant percent of the population) 1st gen German immigrant population. There were concerns they would sympathize.

What was actually happening is many of these immigrants were there to get away from Hitler and Germany as it was at the time, so Germany found most of its attempts stymied instead. But they did try.