>>721094898
The characters bring it up ingame. If Sonic raced them on foot it would be unfair. Because they're all in vehicles they can actually have a chance at beating him. Sonic is participating because he isn't an autist and has a wide variety of hobbies, including an interest in vehicles which meshes with his love for racing. Riders also shows how Sonic has a very competitive spirit. He could beat Jet if he just abused his speed but it wouldn't be beating Jet on his own level, it'd be meaningless.