Legalising stealing is basically abolishing property rights.
But if we're going to limit property rights, we should do so systematically by redistribution, not by allowing ad hoc violations. Predictability is the most important economic benefit of the rule of law.
Additionally you'd create a pressure in society to increase wealth equality / charity and opportunities for the needy / poor, since nobody likes to be stolen from all the time.
Right now the poor are an abstract problem for many, especially the rich, if you ran the risk of being stolen from directly maybe you'd have a stake in increasing wealth equality. Or maybe you'd just hire private security.
It might actually be a better system, or it might not, who knows?
Unless you're suggesting everyone be allowed to steal, you're going to spend as much effort (if not more) working out which thefts were legitimate as you are working out who is allowed welfare.
Not to mention you're replacing a predictable system with ad hoc chaos. (Maybe insurance can smooth it a little, but then that's just further overhead.)
>Or maybe you'd just hire private security.
Private security is largely toothless if they can't hand people over to the police. Even if they throw you out, you can just come back the next day and try again.
Further, it is an exception based on a principle that has historical roots that pre-dates every modern legal system. It is totally unsurprising that some of them have codified principles that can be used in this manner. Especially given that e.g. the principle of necessity in many jurisdictions can be used to justify far worse abrogations of property right than petty theft.