Good question. There is probably 4 or so reasons why they operate off the springboard.
Mostly people figure it looks like they are getting above the bell end flair so they end up with a more cindical pipe. Very true. That's got several advantages on it's own once the log is down.
As we see there is no flair in that picture, though at least not above the ground.
A consideration for height is cleaner wood, too.
I use my axe to clean moss and soil. Unlike 'these guys', my butcher's knife is only the saw. Some of the old joints, you make or pay. (Been there many times) I'm talking a deeper wet belt and not a Dougles-fir ecosystem.
Continued...