Because gravity leaves the brain without blood.
It is an issue of gravity and hydrodynamics. Blood flows due to pumping mechanisms and suction. The heart pumps forward and creates a vacuum inside that sucks the blood from behind. arteries elasticity. Para cooperate with heart avoid congestion, these store excess blood.
Meanwhile, the pressure of the contractions of the muscles around the veins also working on pumping. If we were long standing , blood pools in the legs, and the suction mechanism of heart failure : low blood pressure and brain runs out of sufficient irrigation. That’s when we dizzy and we fall.