I've built a number of hot rods over the years and gradually shifted to crate engines. No muss, no fuss and no waiting along the side of the road with an obscure engine problem or diagnosing coolant in the oil. No tearing apart a finished build and no oil drips in a clean driveway. Same with tractor engines. No loss of $2,000 worth of hay about to get wet in the field while I tinker with loss of compression in a cylinder or whatever.
Yes, skill in rebuilding will head off these problems but unless you do this stuff every day, little mistakes can slip by and cause all sorts of problems. For a few extra dollars I can shift the risk to someone else, like the manufacturer, that has a great product, close tolerances and good quality control.