I'm not super knowledgeable about this so take this with a grain of salt. Some of this might be wrong.
Good modern touch screens, like the iPhone/iPad/Macbook trackpad are capacitive touch screens. An array of sensors emit a changing electric field which is affected by the dielectric constant of nearby materials, and fingers have a different dielectric constant from air. Capacitive sensors cannot detect pressure (although you can fake it a little).
Good modern digital pens, like the Wacom or Pencil, use radio waves. An array of antennas emits radio waves which are picked up by an antenna in the stylus. A microcontroller in the stylus computes the position and angle of the stylus, reads the pressure from another sensor in the stylus tip, and transmits the data back to the tablet over radio.
So you can see that they really are completely different technologies.
You can make a capacitive stylus, and those exist, you can even buy them at the store. However I've never seen a capacitive stylus that can detect pressure or angle at all, and capacitive sensors can't tell the difference between a stylus and your hand, so you can forget about resting your hand on the screen when you draw. The main reason I'd use a capacitive stylus is because I want to use an iPhone or iPad while I'm wearing gloves.