Why don't sumo wrestlers get muscular

What are the physical requirements for sumo wrestlers? No missing six packs?

TL; DR

The fair is one of the most important aspects of winning sumo matches.

More details

The most important physical requirement, at least in Japanese sumo, is height.

In 1994 the Japanese Sumo Association required that all sumo wrestlers be at least 1.73 m tall.

In addition, there is no weight class. Since the other rules, like prohibiting a large number of potential hits and reducing your opponent's allowable gripping area on their belt and what you can physically circle around, limit your options when it comes to a purely physical confrontation, it's more bulk important (this type of goes back to the real-world piece where a bigger opponent generally has the advantage, despite all the 5 foot stories that nothing masters to destroy massive fighters with ease). As one researcher put it when investigating why sumo wrestlers don't die of heart disease (which is actually a question of general fitness):

The basic principle is simple: winning in the arena depends on the dynamics. And in high school physics we learned that momentum = mass x velocity. This is how you build your mass and train you for speed, agility and general fitness.

Fat is easier to add to the body than muscle mass, and so the general method of building the right body involves practices that increase weight gain while also exercising to be able to move that weight.

Wrestlers are typically not allowed to have breakfast and are expected to have some sort of siesta after a big lunch. The most common type of lunch is the traditional Chankonabe sumo menu, which consists of a boiling stew of various fish, meat and vegetables cooked at the table. It is usually eaten with rice and washed down with beer. This regime of no breakfast and a big lunch followed by a sleep is designed to help wrestlers gain a lot of weight in order to compete more effectively.

Trying to compete in sumo while training in a way that limits your mass gain is essentially competing with an arm tied behind your back by training for the wrong metric, like training for a boxing match by You forego hitting power in favor of fancy footwork or compete in a sport that has weight restrictions while carrying excess fat that won't help you in the ring.