Planets orbit in roughly the same plane due to the Lense-Turing effect, also known as frame drag. The gravity of a large spinning object drags space along with it and so objects are drawn to the equatorial plane. The effect itself is tiny but relentless. We now have evidence of exoplanets that have been perturbed in their orbits and so orbit well off the plane. Over time those will be dragged back to the plane of the parant star.

If we look at our own system, the planets deviate from the plane more as we move from Mercury out to Pluto. Earth is the only odd-ball which indicates that at some time in it's deep past Earth's orbit must have been perturbed by some passing massive object. If we go out farther we can even see the effect in the oort cloud, where it's resembles a torus rather than a spherical shell. Planets may form on a plane but there is no reason for them to stay there. In Newtonian physics there is no preferred plane of orbit, but in relativistic physics the spin of the star does create a preferred plane of orbit.