Renewables are hard when your population density is high. The availability of renewable energy scales with the size of the country, while the demand scales with population.

Last year, German onshore wind generated 111.9 TWh, or roughly 320 MWh/km2. I can't find the numbers of onshore wind in the US, but the total energy generated by wind power was 453.5 TWh last year. Divide that by land area, and you get ~50 MWh/km2. Because the US has developed only a small fraction of its wind power potential, new turbines can still be built in favorable locations, and capacity factors are high. Germany has to build new turbines in marginal locations, with lower capacity factors.