They need their hits in the A-box as fast as possible. Even if they could shoot with one hand while running faster, they wouldn't gather up the required points per second.
This is the crux of the issue.
The IPSC shooters are moving as fast as possible while
ensuring A zone hits. It wouldn't make sense for them to move faster because the time difference between their speed walk pace and a run wouldn't equal the penalty of a C zone hit. This doesn't by default mean that nobody can shoot/ move faster via a different method. It just means that this is the proven winning speed/hit combination method in this sport.
What if they changed the rules? What if the A and C zone were combined into a "hit" zone? Might that change everything? Then a new video would get posted about how the best in the world shoot and move (differently than now).
This doesn't mean running while shooting one-handed is superior either, but it is apples/oranges to what IPSC shooters do.
I have shot and hit while actually running offline one handed and made quality hits at 4 yds and under. I can do it way faster than anything 2 handed. I also would never do it in a shooting competition, distances too far, too much penalty for a miss.
I don't have a video, but in this photo when an unknown target faced, I ran offline while drawing and firing and got 2 sternum hits at about 4 yds. My pace was faster than anyone in the IPSC video.
Please don't mis-construe this as me saying I'm better or have a better method than the world-class shooters, not even close. When I say my "pace" was faster, I mean I am running a lot faster than them. My time to draw and hit this target 2x is way slower than they would be...because they are world-class shooters.
I am faster offline with acceptable hits running and shooting 1-handed than
I would be fast-walking, shooting 2-handed, at this close range.
At 4 yds and under, fighting for my life,
for me, I'd rather run offline and get a "C" zone type hit for the first shot. Shots #2-the end of the gunfight, get better as I close distance and/or get to cover and/or get more of an extension and a full sight picture.
For shot#1, I prioritize getting off the X, and any decent hit.