Often the simplest explanation is the best. From the point of view of the Ancients, there were 'Seven stars in the sky": Sun, Moon, Mercury, Mars, Venus, Jupiter and Saturn - the only ones visible from Earth without sophisticated telecopes. Add that seven, like three, five and nine, is a 'balanced' or odd number, and that such numbers have always had a sacred aspect, and you have the basis of an idea. The association of powerful spirits with the stars, which coincidentally add up to a 'sacred' number, could very well have been one of the most common aspects of nature-based, shamanic beleifs and thus made it's way into later religions.

Just a random thought :)