In my opinion, it's because when standing we're engaging one set of muscles constantly. Whereas when walking we're contracting and extending different muscle groups in a cycle, therefore giving muscle groups short bouts of recovery.
Compare moving your arm like a pendulum vs keeping it stretched horizontally.
You will probably find that you lean slightly from side to side or stagger very slightly, with your core having to be partly engaged all the time. Your feet are in the same spot, but your center of gravity shifts and the core is the only thing to counter that.
Meanwhile, when you walk, the momentum of your upper body takes care of a lot of this, you just automatically lean forward and your feet get under you, offloading a lot of that adjusting from your core. Your center of gravity moves predictably and your feet can "plan" to get right below it in the next step you take.
Turns out I had what Dr YouTube said was an anterior pelvic twist (standing slouchy, belly out). Making myself stand upright was actually enough to give me an abs workout. I felt from standing the kind of “burn” you get after a load of crunches.
Even now, a year later, I still have to work up to doing more than ~3 hours standing if I don’t stand consistently.
So standing desks have a great for me. My one concern is that when tired or focused I subconsciously lean at weird angles and put pressure in the wrong places, but hopefully I move frequently enough.