I think I’ve got pretty good grip strength, but am surprised at how many seemingly loose bolts, etc. I can’t even budge without use of a wrench.
Which brings me to this question: What is the average maximum torque that an adult male can generate compared to, say, a 12-inch slipjaw wrench, or some other type of hand wrench? Is the multiplier on order of 100 times? 1,000?
I know the wrench’s length makes a big difference, just not sure how much.
T = F x r
More difference is made, however, by the fact that flesh deforms around bolt-heads, and steel, for the most part, doesn’t.
It’s easy to get a feel for it if you remember the units of measure for torque. Now commonly referred to as pound-inches, formerly inch-pounds or currently pound-feet formerly foot-pounds. It works as the name implies. If you have a wrench 12 inches long sticking straight out parallel to the ground and hang a one pound weight on the end of it, then you are exerting one foot pound of torque.
A wrench multiplies your strength many times not only through the mechanical advantage of a lever, but because you can use your body weight as well as arm and back strength to push on it. Grip strength comes primarily from your hand and forearm muscles whereas using a wrench allows you to get much more into it. Plus nuts are usually relatively small things and gripping them is difficult in the first place. Imagine our one foot long wrench again with the one pound weight hanging off the end of the handle. Do you think you could hold it parallel to the ground by gripping the bolt it is attached to? I suspect it would be impossible using fingers only but not so bad if the bolt was long enough for you to wrap both hands around it. So it all depends on a number of factors, but this little thought experiment should help you get a feel for it.
I agree that grip has more to do with things. Most of those “loose” nuts you could remove with a nutdriver, which isn’t a torque multiplier. That said, a longer wrench does increase the amount of force on a nut by a very large margin.
As Nametag said, to your padded fingers, a hexnut may as well be round. Do the same test with a wingnut or thumbscrew, though, and you’ll do a lot better. We aren’t born with a 3/8" drive in each palm, so we have to make do with easy-to-grip tools such as a Screwball®. Some socket sets also come with a hockey-puck shaped driver. Even a regular screwdriver will let you impart more toque than bare fingertips.
Heck, I knew some Nun’s who could lift a 7th grade smart ass clear off the floor with just thumb and forefinger gripping their ear.
I agree it’s not so much the lack of torque generated by your wrist, but the small amount of torque your fingers will put upon the bolt before the fingers deform. The deformation quickly leads to slippage or damage to the fingers.
After all, using a simple screwdriver, which doesn’t increase torgue at all, I can turn many screws that I am completely unable to turn with my bare hands.
Hmmm… I have personal knowledge of this! Sometimes when playing with a weld force gage, we see can get the most force between the thumb and index finger. IIRC 20 pounds-force is a good score.
Oh, but how I wish. 
A nut driver is a torque multplier. Say the nut driver is for a #8 nut. That’s a lever arm on the points of the hex of about1/8". The nut driver handle has a radius of about 3/8", a multiplier of three. In addition the driver gets a good, firm grip on the nut which your fingers can’t do and you can apply more force on the driver handle than on the nut.
Well, sure, it increases torque slightly (although not as much as you suggest, if we move beyond bitty little #8 nuts into larger sizes). But its impact on torque is neglible compared to the increase of torque one gets from a wrench. My point was just that the biggest reason seemingly loose nuts can’t be removed by hand is due to lack of grip, not lack of torque.
This 16" torque wrench goes up to 600 inch pounds. I believe the figure for hand tightening is 7 or 8 inch pounds.