Balance reason: would you use a 2nd level spell slot to cast misty step to move 30' if you provoked OAs? The most prevalent use of that spell that I've seen in-game is to get away from enemies without provoking OAs.What?
Then why can't you cast a spell and withdraw? Casting the spell isn't relevant, as that's not a trigger for an OA, just like you can walk circles around someone and not trigger an OA. Only leaving the reach is. Your in-fiction explanation doesn't explain why leaving reach via teleport doesn't trigger the OA.
In-fiction reason: The physical act of withdrawing, moving your feet and shifting your body to change postion and direction, leaves openings in your defenses. If you maintain defenses while you move (disengage) or if you maintain defenses while you cast and teleport, you don't provoke.