Because when private assets come into the public domain, where there is infinitely more capital, those producing assets become worth multiples more. Future earnings are figured into today's price. 100% of 2B or 71% of 10B, which is better? Plus, it's easier to raise capital for expansion/development/mergers/acquisitions. But in order to go public, there have to be a certain number of stockholders/a certain % of the company for sale.