I thought it had something to do with the fact that they are facing head-on with rivals and maybe something with their psyche in the end, and the answer is extremely boring = \
According to the rules of the Japan Sumo Association (hereinafter ASYa), wrestlers are prohibited from driving a car. For example, in April 2007, wrestler Kyokutenho Masaru (旭 天鵬 勝) was expelled from the tournament and demoted after being involved in a minor accident. Another wrestler, Toki (と き), was expelled from several tournaments after one person died in an accident involving him.
Regarding Mitoizumi's accident. It was an accident led (how unexpected!) With Mitoizumi Masayuki (水 戸 泉 眞 幸), who in May 1985, at high speed, after losing control of a car, received numerous cuts on his face and injured (already unhappy) knees, after which he I had to take sick leave and miss numerous tournaments, which is why I was demoted.
In fact, they cannot get behind the wheel for two reasons:
1) Because of their large sizes, ASYA came to the conclusion that they are not able to drive a car normally;
2) The main reason was that ASYa was indignant due to the fact that due to constant injuries on the road, wrestlers miss tournaments, thereby violating the regulations.
Japan is Japan.