The uppermost parts of the atmosphere are very dynamic, with changes in solar activity causing them to contract and expand on a scale of hours. And this is very significant for the amount of drag felt by re-entering objects. Because of this, if you slowly deorbit something naturally through drag, you are not going to be able to pick the destination, or even know where it ends up until very close to it falling down. So instead you intentionally lower the perigee so that it deorbits in a controlled manner.