Because the goal is short term profit, not long term business success. It makes absolutely no difference if the company survives the process or not, what matters is that the PE firms extract their money from the process.
I think if you actually reflect on the matter you would realize that PE firms need to be able to sell the business in order to make money, and that they do in fact sell the business for a profit in the majority of cases. The extremely rare cases of yore where you could buy a business for less than the value of its assets and simply sell off the assets and leave the carcass for bankruptcy are long gone.
