Because if you go that far you deny the whole concept of individuality and humanity even, up on which all of our moral and legal system were built. What are we but cogs in a machine?
It's not that there isn't any logic in it, it's just not an useful thing to contemplate about while you obviously still cling on to your sense of self so far as to having an argument with others.
And, on the otherside, we are a funny animal that takes comfort in the fact that we control our own destinies so, even if we don't - we'd like to empower ourselves as if we were in some manner. At a basic level our misunderstanding might be the fact that the ability to choose equates responsibility - without any responsibility (good and bad) your choices have no power - choosing to not do the shitty thing lets you avoid a penalty and so you can be happy with yourself. This equation of choice and responsibility is why punishment is so necessary (both big-j Justice like cops jail and the like - and little-j justice like being shunned, not getting a raise and failing to find an equal partnership in a relationship) so responsibility exists to add meaning to our lives. In fact denying it to people permanently (i.e. the developmentally disabled) does come with a strong cost to their sense of self - a lot of folks who live in support systems can struggle to define themselves, but, at the same time, neurodiversity means that different folks can find contentness in different ways.
It's all terribly complicated I think.
I don't follow this.
We don't blame people for actions when we think that they don't have control over these actions.
If someone has their first epileptic episode while driving a car and they kill someone because of it, we understand that they had no control and don't hold them responsible for the accident. Why should it be different if the causes are more complex and harder to trace back?