Because the sun is many times bigger than the earth. Even if the sun were thought to be smaller than the earth at that time due to a lack of knowledge, its size would still be quite large, and its light would surely fall directly on the earth. In order for a shadow to form, the path of the light source must be tilted. Otherwise, the light-receiving card must be tilted. So now that the light source is not tilted, the only thing that remains is the tilt that accepts the light.