Why fingers in human fists get folded in absence of neural stimuli?
I would assume that as a result of the lengths of the tendons and the arrangements of the various flexor + extensor muscles, the slightly folded positions (curved) fingers, hands, legs, etc. are the "default" positions, in which both antagonistic muscles are in their relaxed state (thus without neural stimulation)
The evolutionary benefit of the slightly curved fingers, hands, legs, etc. may have been more relevant in our primate ancestors than it is today