SCENARIO A:
Let's say you bet $200 (your stack is $800). Next guy calls. Next guy calls. Now what if you had the chance to raise again, and essentially start a new round of betting? Obviously the rules prevent this. You can't just keep starting new rounds of betting.
SCENARIO B:
Now how about if you bet $200, next guy goes all-in for $201, next guy calls $201. Now it's your turn again, but if we give you the chance to raise again, we essentially have the same thing as scenario A. You would be starting a new round of betting with no real raise against you, for all intents and purposes.