For convenience sake let's compare a forklift truck to a car as shown above. The first thing to note is that a forklift does not have any suspension of any kind. Imagine a car without any suspension. No springs, no shock absorbers, in fact nothing: just rock solid. There would however be some suspension as the car uses pneumatic tyres so let's change the car tyres to solid tyres.
Still the analogy is incomplete however as a car is much wider than a forklift so imagine a popular car but about the same width as a forklift. In addition to all of this, the truck, especially when stacking, is higher than a car and has a higher centre of gravity.
If the car really behaved in the manner described above then the first time it encountered any sort of sharp turn it would obviously tip over and yet that is roughly what you are driving when you operate a forklift. The major difference of course, is the truck does not go anywhere near as fast as a car!