Because it would greatly increase the cost and risk of investment. Think not just for billionaires, but for anybody. Imagine somebody buys a couple tens of thousands of dollars of a stock as part of their retirement, that company does something bad, and now not only do they lose their investment but they lose the rest of their retirement also.
I am all for wiping out shareholders, especially big ones, when a company does something super stupid. There should be an incentive for shareholders to hold companies they invest in accountable.
But suggesting that company owners become personally liable for the actions of those companies, especially when those equity owners have little or no control over the decisions of the company, that is a recipe for disaster.