No, you're not missing something. It is a choice between bumpy rides or clipping through the ground and shooting you skywards. The developer made the wheels with less polygons (3D points) so that there's less chance of wheels clipping through the ground at high speeds. Gmod calculates the position of the wheels everytime a polygon touches the ground. Having too much polygons will overload Gmod with actions, resulting in skipping some.
(you can tune up your vehicle with ENOURMOUS numbers and you'll see that you are flying in no time.
The reason why your tank has a stable ride is because of the low speed and the amount of wheels it has. Those extra wheels will compensate the possible mistakes that other wheels have.
(hope you're still reading this as you posted it about a month ago.)
Hope i helped you

the developer should add an option for spherical tires so that it has an even smaller chance of clipping

You can't make 100% spherical tires, you always need polygons. You can make them so that the middle of the axle stays a given height above the ground like hoverballs that don't get lower than the amount given. Then you can make the tires have no hitbox and have a stable ride.