Veins in human legs only have one-way valves in them and they work when the skeletal muscles around them contract and release. The flexion/relaxation while walking pumps blood back to your torso. When standing still, you don’t get that pumping action naturally, which is why people can walk miles without any discomfort, but when standing in a place for even a few minutes makes them uncomfortable. Legs are sometimes considered a “second heart.”
Seasoned military personnel can stand at ease for long periods of time because they are actually swaying back and forth very slowly in micro-movements to contract their muscles and relieve the tingling and numb sensation you get when you keep standing for long periods of time. You too can do that if you ever feel lightheaded.