Magi become more powerful and less human as they use their gifts. The physical changes are the most obvious: one mage may grow increasingly translucent, while another will catch fire without burning.
But their minds, their personalities, are slowly written-over, changing them to the point where, in some cases, they remember nothing from before they started learning. New phobias, new obsessions, replace what was there before. Eventually, though, they all disappear. Into thin air, as it were. The Priests claim that it is their gods taking them to their reward, but that doesn't explain the elementalists or the sorcerors.
In reality, they are all just shunted into the spirit world, able to see the material, but never to effect it without mortal assistance. This is where the Sorcerors get their power, by forcing spirits to do their will. Not that they know this, they think the power is entirely in the sigils they draw.
Sorcerors don't do well once they go to the spirit world...