Because even IF they're not as accurate as a calibrated torque wrench, using them on lug nuts has NEVER been a known problem to me or anyone else I've talked to over the years since Volvo supplied them to us 10 yrs ago.
Why don't people you know use them? I don't know. I've also heard some say what you're saying but then I hear ALOT of BS in my business so I take much of it with a grain of salt and do what I know works from my experience.
All I do know for a fact is that the tools that Volvo has supplied doesn't give me or the other techs in our area any problems.
Why don't I use them for other applications? For one, these days almost all bolts that really require equal torqing at all all get angle torqued (unlike years ago) SO one then needs to be exact when doing critical work like engine repairs (say torquing a cyl head) because it starts out at low torque (not impacted) and then gets angle torqued to incredible tightness. It's seemingly so tight that one's almost afraid that the threads might give way SO accuracy is very important there. I also don't own a set of torque sticks as sold by tool distributors. I use the tools that Volvo supplied us with and is required, mentioned in all their OEM service manuals. Maybe they're better than what one can buy elsewhere?
Volvo requires the lug nut torque sticks as a Volvo shop tool and since they've been in use, I have yet to hear of a specific problem with lug nuts or warped rotors from their use.
Again, I use the one for 700s (65 ft lbs) on my 745T everytime and I've owned that car 8 yrs now. As you might know, 700s were notorious for warped rotors in their day so accurate torquing was essential. My rotors don't warp.
The 850s on up have not been as critical on accurate wheel torque as the 700s were and although I don't condone it, I've seen many a non-Volvo tech (one who doesn't know better) use a standard impact socket on 850s, etc w/o the rotors ever warping. How would I know? We have regular customers who only bring their cars to us so I'd know if one of their cars had a warped rotor problem after someone rotated wheels.