On Groundhog day, if the ground hog sees his shadow, why do they say that we get six more weeks of winter?

If the groundhog  sees his shadow and it's sunny, it's supposed to be six more weeks of Winter. If it's cloudy and he doesn't see his shadow, we're supposed to have an early Spring. That's if you believe that stuff but that's the folk lore.

If he sees his shadow he is frightened by it, and goes back into his house for six more weeks

Everyone knows it's just superstition believing in ground hogs.

The way to predict the onset of Spring doesn't have anything to do with ground hogs. If you want to know that, you have to rely on regular hogs seeing their shadows!