It is a compromise between comfort and practicality. Yes the off-road cars would have a smother ride if they have constant rate springs, but then they would need a much larger suspension to accommodate all the play.
Say two cars with similar mass on two surfaces with same speed:
1-a well leveled, smooth track.
2- a coarse bumpy off-road trail.
The first car is expected to hit obstacles of say 1 inch height and the second 3 inches.
In a spring with constant ratio k, F= kx and the jerk or acceleration the bump will give to the car will be F = m*a therefore a = f/m.
but if the spring in the second car is to absorb a 3inch obstacle with the same constant k it needs to recoil 3-5 times x or even more depending on the speed, and this will call for a very deep spring box.
So to optimise the performance of the spring they design the spring with a gradually increasing stiffness, e.g. $F = k*x^2$
This way the suspension can take a much larger range of bumps and at the same time is comfortable when meeting small obstacles and tough enough to take the big ones and still not bottom out.
The way the accomplish this is to build the spring with gradually thicker coil diameter, or use spring leaves, or design the geometry in a way that the play of spring becomes stiff as it recoils.