I think its because tori is quite heavy, and its difficult to control how he displaces his weight, we as humans have an more complex range of movemnt which can also be changed in power/speed, as a result we are able to create movements which do not displace us as much as movement in toribash displace a character, because uke/tori only have 20 joints which move at a fixed velcoty power and range it is extremely difficult to control your centre of mass to remain balanced