Auto conversions & gear boxes

I notice that most auto engine conversions use a gear box between the engine
and the prop. Why is that? Is it because an auto engine's peak HP is too
high for a prop to swing? Is it because auto engines weren't designed to be
pulled around by their crankshafts and don't have proper thrust bearings?
Both?

Are there any auto/motorcycle conversions that don't require gear boxes?

Dave