What you need to remember is that all of this applies to probabilistic arguments with probabilistic results—of course deductive reasoning can be done backward. However, when evidence is presented as contribution to a belief, omitting some (as you will, inevitably, when reasoning backward) disentangles the ultimate belief from the object thereof. If some evidence doesn’t contribute, the (probabilistic) belief can’t reflect reality. You seem to conceptualize arguments as requiring the outcome if they’re valid and their premises are true, which doesn’t describe the vast majority.
What you need to remember is that all of this applies to probabilistic arguments with probabilistic results—of course deductive reasoning can be done backward. However, when evidence is presented as contribution to a belief, omitting some (as you will, inevitably, when reasoning backward) disentangles the ultimate belief from the object thereof. If some evidence doesn’t contribute, the (probabilistic) belief can’t reflect reality. You seem to conceptualize arguments as requiring the outcome if they’re valid and their premises are true, which doesn’t describe the vast majority.