Q: Why do off-road drivers pick the 18-inch tire over a bigger diameter?

Because a smaller wheel means a taller sidewall. The 285/75/18 puts the most rubber between the rim and the ground of the common Cybertruck fitments, and that extra sidewall soaks up trail impacts while letting you air down further for grip. It also puts more cushion between a sharp rock and your rim. It is why serious overlanders lean toward 18s. On road you give up a hair of steering sharpness for a more compliant ride.