Why isnt sleeping for 8 hours a risk factor for blood clots but inactive lifestyle, long car rides/flying are?

When you sleep, you are not immobile for the entire 8 hours whereas in a long car ride or a long plane ride, you are more immobile. When you sleep, your legs move during the 8 hours except when you are in REM sleep which is not the whole time. In a plane, moving around during the flight, when able, is helpful to try to prevent clots but this is more difficult to do in a car. Frequent stops helps!