Because if I google 20/400 vision most things I'm turning up seem to indicate that it's perfectly possible to have 20/400 without corrective lenses.
Not least it would be a mostly useless scale if 20/200 was everything from -2.5 diopters and up. That's not even out of the first level of myopia (0-3, 4-6, 7-9, 10+)