Block and beam floor is required in clay to avoid ground heave.
Although clay can dry out it and shrink it can also get wet and expand, this would then 'push' on the floor if it was a floating slab and crack not just the slab but put pressure on the footings and possibly damage the whole structure
It can be overcome with clay board that compresses, but as the clay shrinks back it can leave voids. As its a garage the floor will be designed to take a lot more weight than a domestic house floor and voids would potentially be a bad thing.
Making the oversite and concrete slab thicker then becomes cost prohibitive, hence the block and beam or suspended floor
A lot of cracking in buildings isnt due to subsidence but actually ground heave
Of course if you can chat up the builder with a suspended floor and a bit of extra dig/ design/ hide from building inspector you could have a nice pit to work in under the car
