I think the fear (potentially unfounded) is that the handle might slip out of my hand and then result in me dry firing the bow (never a good idea, it can break the limbs and cause injury) with the cocking handle free spinning at a very high rate of speed (more injury potential). I hunt in the evenings, so it is dark by the time I descend from the tree stand. For me it is just easier, faster, and safer (?) to fire an old beat up bolt into the ground or use a de-cocking bolt.
Doing this also allows me to test my accuracy (and equipment) in low/no light conditions because I'll pick a ground target or area that I can identify (so that I can later find the bolt). I use lighted tracers on the bolt nocks that are supposed to activate when fired (not always 100% reliable). It is just one more way to keep the skills up and confirm that I'm doing things right under simulated conditions.
The bow is a secondary concern. Decocking manually, also opens up the door to degloving your fingers, if the string slips uncocking a crossbow.