Monks are supposed to have supreme control over their own mind and body. One can argue that all things that affect one affect the other. That is why they have great strength without the use of weapons. Their intense focus allows them this. Similarly, they use the power of their mind to create physical change in their bodies, thus healing themselves or hardening their bodies to danger. Notice that, for the most part, their abilities only affect themselves. It's a inward mind over matter concept, which is different from typical magic that is projected outwardly.

Personally, I find monks boring and I hate using them. Being able to derive power from your mind and body is cool in concept, but in a video game it basically means you don't need to upgrade them at all and that's pretty boring. I like getting shiny armor, powerful weapons, and mystifying magics for my characters.