Because a) it would be awkward to remove and replace the laptop from a slot on your back, and 2) sitting down in any chair with a back would be uncomfortable at best, and potentially damage your laptop.
I dunno about drawing attention. I've heard enough stories of physical pentesters getting into secure areas using the "wear a hi-viz vest and a hard hat" strategy. The outfit seems a little like Douglas Adams' "Somebody Else's Problem Field". People seem to be blind to the stereotypical "workman" getup.
Ha! I used to be a land surveyor before I was an engineer. I had one of those vests and yes, I did put a laptop in there a few times during lunch breaks when I didn't want to leave the laptop in the truck.