While I think it is true that mistakes cannot be undone, it's also the case that most mistakes (even fairly bad ones) are not a sentence of doom. There are still choices to be made in the future (if you think there are choices) and you can always take the stoic route which is imo quite useful regardless of the question of free will.
There is no "why" -- it happened and was not selected against; it may have been adaptive and that is a kind of why, I guess. I think it would be better to be clear about this in science communication.
Of course there is a why. It happend and not only was it not selected against, presence of a tail was selected against. Meaning it was, or came along with a beneficial adaptation. That benefit is why it took over our gene pool and why we don't have tails now.
