That's mostly because people confuse
training with
habituation.

Trying to get a dog to accept it by doing it over and over again is not the same thing as shaping a completely new and positive behavior.
I work a lot with dogs (and different exotic mammals as well) and once trained a rescue German shepherd who was a known biter to do this. He would litterally try to bite and lunge when the owners needed to clip his nails, they had to be like three people just to wrestle him down.
So we started from the beginning by just clicking and treating for being able to have our hand NEAR his paws. Over a week (we take it very slowly with these dogs) we then shaped it into being able to touch his toes and nail whilst he was still relaxed, and eventually into him placing his paws in our hand by hos own. When they're comfortable with this, you introduce the clipper at a distance. A friend of mine actually turned the sound of the clipper (cutting into a twig) into a "clicker"/secondary reinforcer itself.
After that it's just a matter of shaping it to the point where the dog is relaxed with you tapping the clipper to it's claws, holding the clipper around a claw, putting a bit of pressure on the claw and eventually clipping, and rewarding small approximations all the time.
This is how you train a behavior like this, and i have actually not seen this fail with any dog so far.

Here's a quick vid that explains the process, even though you might have to take it a lot slower with other dogs. (And this is why you need to start long before it is time for a trim)
http://www.youtube.com/watch?v=MuXwKqXTBsE