Because having a simple and logically consistent system to decide what is right and wrong in any given situation absolves one of the responsibility to make complicated moral decisions on their own merits.
I understand libertarianism and I very much appreciate that it is built on a foundation of reason. However, being an imperfect being, a wise man tempers his reason with empiricism.
> Because having a simple and logically consistent system to decide what is right and wrong in any given situation absolves one of the responsibility to make complicated moral decisions on their own merits.
But libertarians are the ones who want to take direct moral responsibility for their circumstances; it's statists who seek to outsource that responsibility to distant formal institutions or systems of pre-defined abstract rules.
You may want to take direct moral responsibility for your circumstances, but that would be very dishonest. Whether you like it or not, you are part of a society and the very existence of society makes your circumstances possible.
Every human being, like every other discrete thing that exists, inhabits a particular context; the question is not whether an individual takes responsibility for himself with respect to some abstract, Platonic universal, but whether he takes responsibility for himself with respect to the particulars of the world he actually inhabits.
This includes choosing what trade-offs to make in the interest of maintaining or creating specific relationships with others - this is what society is. But what you're referring to as "society" is merely a logical construct, abstracted so far from the actual circumstances of people's lives that it does not represent anything that really exists.
Actual societies only emerge from the substantive interactions of particular human beings. Suppressing those individuals' ability to freely negotiate the terms of their relationships by forcing them to adhere to generalized rules, promulgated in the abstract, actively damages society.