Because one of the great viscerally satisfying part of watching a sumo wrestling match is the strong initial clash. Side-stepping the charge can be a thing that

Because one of the great viscerally satisfying part of watching a sumo wrestling match is the strong initial clash. Side-stepping the charge can be a thing that makes you more likely to win the match, but it makes the sport of sumo wrestling less fun to watch and less prestigious/important/etc. Basically, it's defecting in an iterated prisoner's dilemma. "Considered beneath the dignity" is just shorthand for "there are social enforcement mechanisms aimed at preventing this behavior".