Because they _are_ boring. I have no idea what to think of the GP other than to assume they've never actually worked a blue collar job. I have. They suck. They're boring. They are the definition of boring work.
Here's a list of manual labor jobs that I worked during my teenage years. This list will not include any job that I worked in my 20s. If I added all of the manual labor jobs that I had in my 20s I'd be here for the next hour.
* Picking corn/green & soy beans/tomatoes/peppers/berries by hand (this is summer and fall only, mind you, so not only is boring it is also hot -- even when you start at 5:45AM, you get very little reprieve working the fields because you're there until 11AM or noon).
* Working a farm stand, selling aforementioned corn/green & soy beans/tomatoes/peppers/fruit (moving up in the world!).
* Flipping burgers / running a fry station / running a fast food cash register / washing dishes in a fast food joint.
* Delivering pizzas.
* Delivering printed materials (hard to explain, friends dad had a printing business, I ran deliveries for them for a while -- loading, unloading, driving. Driving for a living is easily the most boring thing that can ever happen to you, IMHO).
* All manner of construction -- but mostly roofing and framing.
* Working the end of a glass plant factory line, inspecting glass television screens for defects. OT was in abundance here and I would often work 16 hour shifts doing this.
* Waterproofing basements / dealing with broken sump pumps / digging out deteriorating septic tanks.
* Throwing boxes at a FedEx.
This list represents ages 13 (maybe 14?) - 19 for me. 3 of these jobs were things that I did while attending University (though I left and didn't return until later in life). It doesn't get much better until 22. At 22 there's some manual labor that is disguised as technical work (military). A lot of this is or was seasonal work. That was the nature of manual labor, when and where I grew up. I'm sure that hasn't changed too much.
All of these are boring jobs. They are mind-numbing in ways that are genuinely hard to express. If salaries are dictated by who is doing the most uninteresting work then we have things _completely_ backwards, IMO.