Off the top of my head, I would assume that it's because in nature solar systems form by dust gradually accumulating into planets. This is different to just plo

Off the top of my head, I would assume that it's because in nature solar systems form by dust gradually accumulating into planets. This is different to just plonking a brown dwarf right into the fully formed system. I also think that the solar system isn't entirely stable. The orbits of the planets do influence one another in a way that isn't in equilibrium. It just happens very slowly.