The reason is quite simple. They find the best artwork position by eyeing the room, rather than by taking on a mathematical approach and measuring the exact centre of the wall. This is best executed when all the furniture is in place, so they can get a feel for the best placement of the art.
If you are a bit OCD (like me), watching this unfold for the first time may be a bit uncomfortable. Our stylists put the artwork on the ground, leaning it against the wall they want to hang it on. Then they take a few steps back, tilt their head left, tilt their head right, pick a spot and without much hesitation hammer the nail in. And then the second one.
Want to know the best thing about this? 99.9% of the time it looks absolutely perfect.