They assume the orbit is constant, having been in place since the inception of the Solar System. They assume the distance from the Sun is maintained by centrifugal force, pulling away from the Sun. They assume the rotation of planets is a constant, and having no explanation for rotation ascribe it also to motion having been in place since the inception of the Solar System. Humans view the result of many factors they do not understand, and ascribe this result to inappropriate causes. They are wrong on all counts, but as the Solar System does not change in its motion before their eyes, this is not often up for debate.
Rigid minds have no reason to change.
Comfortable theories have no uncomfortable challenges. The orbiting planets are indeed caught in the Sun's gravitational field, but there is more than gravity and motion at play in maintaining the distance they do from the Sun. The orbits are scarcely fast enough to create a centrifugal force strong enough to keep them at a distance from the Sun. Planets do not drift into the Sun, in the main, due to a repulsion force generated in both bodies. Where the force of gravity is constant, and steadily pulls a smaller object toward a larger, a repulsion force is generated between objects, and only becomes strong enough when the mass of the two objects is sufficient.