Question

why vehicles are provided with round tyres only and not any other shape

Answer 1

because the rolling effect of the round wheels gives us less frictional force so the Tyre doesn't wear out. and also if other shapes were given the vehicle won't be stable

Answer 2

there are  2 main reasons....
1) for speed...
2) for comfort ride
it they r in any other shape like square or rectangle.....they have points i mean edges that dig into the road........this makes the movement slower..
and with this shapes rolling is definately not as option