Since you said "seriously," I'll answer. Because otherwise no one would invest in any business that they were not personally operating. Suppose you own some shares in a mutual fund. Is it reasonable to hold you personally responsible for actions of businesses whose stock you hold? Exxon has millions of shareholders. Is it reasonable to track down each one of them and shake them down for the costs of the Exxon Valdez cleanup? How many of them had any *control* over Exxon's corporate policies?
Limited liability is an essential component of having any sort of industry beyond the cottage level. How likely would you be to invest $10,000 in a startup business if you knew you would be personally liable for all the business's debts if it failed?