It is easier to close off federal property than it is to create exceptions. The virus spreads fast, therefore we have to react fast. Anyway, social distancing does work. Look at Ohio, which has been ahead of the curve in terms of tackling covid-19. The infections (per million) are the lowest of any state. You can see this in history as well, just do some research on the Spanish flu. You'll find that those regions which implemented social distancing had the lowest amount of deaths and infection.