Because in every nebula that creates a proto-star, the remaining part of the nebula, the accretion disc, that doesn't become part of the star can condense and clump together to form planets around that star. Today, as astronomers continue to pierce the skies, they find young stars surrounded by their maternal nebula in the same fashion as was theorized with the formation of our own Solar System.
You're pretty tricky in doing your homework, being answered here in Yahoo! Answers but this is better that those 2012 freaks. At least, I have something to waste my time onto.