Your calves are your second heart.
Gravity pulls fluid down. When you stand — or sit — for hours, the calf muscles that normally pump fluid back up your legs go quiet. Fluid pools, and by the end of the day your legs feel heavy, tight, and tired.
That heaviness isn't in your head. It's hydraulics. Walking and bending act like a second heart for your lower body; long, still hours switch that pump off.