Your legs take the most and get the least.
A hard training session. Ten hours on your feet. A long flight where you barely stand up. By evening your legs feel heavy and swollen, and the next morning they still haven't reset.
The problem is that resting is passive in the wrong way. Blood pools in your lower legs when you're still, and it pools when you're upright all day too.
What actually helps is movement through the leg when you aren't moving, and that's the one thing you can't do for yourself. And the longer you wait, the worse it gets.