it would mean ids going inside the hundreds and possibly thousands, therefore voiding most of the convenience this would offer.
another thing is that it's impossible to assign an id to a player - it is done automatically and internally by samp. that means we wouldn't be able to see it in the scoreboard (tab), therefore causing even more inconvenience.
and the purpose that it would give can be replaced for most players by just using a unique part of their nick, like "vert", "moo", "crom", "lip", "mart", "3ld" etc.