Because Falcon9 is a big launcher taking a serious payload into orbit. This means that the first stage has to have very large engines - so large that they can't be throttled down to hover, they have to kill velocity at the point of touchdown.
It's the difference between slowing your car down gently to stop at a red light, and having your car going at max speed and slamming on the brakes as hard as you can at exactly the right moment so that you will eventually stop at the lights. One is easy to do, the other will most likely get you killed.
It's the difference between slowing your car down gently to stop at a red light, and having your car going at max speed and slamming on the brakes as hard as you can at exactly the right moment so that you will eventually stop at the lights. One is easy to do, the other will most likely get you killed.