For F-1/OPT there is no 'pending immigrant visa case' status that lets them remain in-country after OPT expires.

A crazy number of people adjusting status, most notably DACA recipients, are adjusting in the USA (despite the much longer wait) because leaving the country may trigger a very long re-entry ban. This can be avoided through advance parole, but turns out, there are a limited number of things for which that's granted like employment and education and US consular visits don't appear to be on the list. So "just leaving the country" is a guarantee of your own banishment. In fact that's probably part of the reason why they picked this policy in the first place.