Why?
http://i29.tinypic.com/2ynkqdh.jpg
Of interest is the cartridge and the cast below it. Notice where the case mouth of the cartridge is compared to where it would be in the chamber. Also notice the transition(step) from the portion of the chamber where the neck would be to the freebore is tapered, not a square shoulder.
Now suppose the neck is too long on a loaded cartridge, the tapered transition would crimp the mouth of the cartridge on the bullet and without room for the neck to expand and release the bullet....Kaboom! The powder gases will take the path of least resistance, back in the action and your snot locker!
Bill