wheels of vehicles are in the form of rings rather than discs.why?

Moment of inertia:

It gives the measure of inertia of a body which is in rotatory motion.

Ring:

Moment of inertia = m r²

​Disc:

Moment of inertia = $\frac{1}{2}m{r}^2$

The moment of inertia of ring is larger than the disc. Hence, it has more ability to continue its state of rotation.
So, the wheels are made in the form of rings rather than discs.