When you bring the tiny guy onto the field, it would become obvious what you're about to do. And you only have 3 tries to add up to 10 yards gained. It would be very hard to average over 3 yards of forward progress past the line of scrimmage with the other team fully aware of what's coming and lining all their own big guys up front-and-center to block the flying little person (or interrupt the hammer-throw process, which would take a few seconds to set up after the snap).