Lots of reasons for shooting high, as already pointed out, but another is a different sight picture. When looking down on a critter, it's profile is smaller than when looking at it broadside from the ground.
For example, lets say a deer is about 18 inches top to bottom and about 12 inches wide. When viewed from the ground, you see the full 18 inches of height. When viewed from above you see nearer 12 inches of body width because, in addition to the deer just being narrower from side to side, you may not see part of the off side of the top of the back or the near underside of the critter.
So, if from a tree stand you hold just under the front leg, you're in effect holding about 6 inches higher on the deer than if you were standing on the ground. Now, my numbers may be exaggerated a bit, but the principle holds true. Just need to lower your point of aim.
Too, the deer appears farther away when viewed from above because the hypotenuse of the right triangle formed by the angle of the shooter to the deer is longer than the horizontal distance to the deer formed by the base of the right triangle. So, in reality, the arrow travels a little farther to reach the deer when shot from above than if shot on level ground. All other things being equal, this would tend to cause a lower hit. However, the perceived greater distance to the target likely causes the shooter to hold higher, probably overcompensating a bit, resulting in a high shot.
In sum, two factors contribute to overestimating the distance to the critter, which can lead to overshooting. One, the deer appears farther away when viewed at a downward angle, because the distance is actually farther than the level ground distance. And two, because one sees less of the deer's profile from above, it appears smaller and thus farther away than it actually is.
Bending at the waist doesn't do much for me. I mostly shoot from a sitting position and don't worry about it. In practice, not bending tends to increase one's draw length a little, which in turn increases the draw weight and arrow speed. However, that difference is so small for most folks as to have a negligible effect at close range. Not enough to worry about IMO.
However, not bending can also change your vertical alignment/sight picture, which I think can be more of a problem.
In any case, where you hold or your alignment or range estimation, the key to solving the problem is practice. Just need to find the gap and shooting position that works for you. No one position works for all shooting situations.