Why is the solar system planar?
Seemingly planar solar system is not planar. The orbital planes of planets are not the same. Likewise, orbits of moons about planets are not in the respective orbital planes.
With reference to our ecliptic (orbital plane of Earth), the inclinations of orbital planes of Mercury, Venus, Mars, Jupiter, Saturn, Uranus and Neptune (in degree) are 7, 3.39, 1.85, 1.31, 2.49, 0.77 and 1.77, respectively. The space enclosing all these orbits is like a pancake, with Sun near the center.