I think the question is really “Why bother with Avada Kedavra?”
Both the Imperius and Cruciatus curses can also be used to inflict death indirectly (curse somebody in such a way that they can’t or don’t stop a 16-ton weight falling on them, say).
The advantage of Avada Kedavra as a tool for killing is that it’s instantaneous.
Almost no protection can stop it, it complete its work in seconds, and once it’s done, there’s no recourse.
The indirect methods you describe could all be deflected or reverted, but a blast of Avada Kedavra is absolute. As we see in canon, Sectumsepra can be healed (albeit partially), flying objects could probably be diverted with Protego (or just duck!), somebody could lift Petrificus Totalus or fish you out of the lake. I wouldn’t call any of those as “simple” as a quick blast of the death curse.
Of course, that’s also why you’d want to be sparing in using it. If you hit the wrong person with a creative method, you could undo the damage. Won’t work so well with Avada Kedavra. See Why don't Death Eaters always use Avada Kedavra? and Why were the Death Eaters so lenient during the fight at the Department of Mysteries? for more discussion of that.