Would sewing in the pleats or attaching belt loops not take away from the versatility of the kilt if this was an item of clothing that could be used as a shelter, bag, etc.? I can see how gathering the pleats in a similar fashion on a daily basis would create natural creases/pleats in the wool (not to mention that I highly doubt they were washed all that often).
I tend to favor the thought that the paintings show more of a "staged" and formal wearing of the kilt than a representation of how the average Scot would have wrapped and worn it. My outdoor gear/garb certainly hangs and fits differently than a suit.