Actually it is. We just don't have the tech today to do it, and atmosphere tends to interfere with the concept.

The plasma is generated from the gases of the atmosphere in between the prongs, and is held in place while it builds by magnetic fields. The plasma is then shoved along its merry way by another magnetic field towards the target. Now, the matter of "how does the plasma hold together" pops up. Well, there are two ways. The first and most likely is that a current is induced into the plasma to coax it into a plasmoid state, which can hold itself together without any outside help. The second, and less likely, is that the gun emits a magnetic bubble, but if that were the case, then you'd have plasma bolts going haywire when more than one plasma weapon is used in the same area.

All of this happens faster than you can blink, by the way. Otherwise, we'd see the weapon charge for each shot.

So, how much do you really know?