An IPO is a fundraising event, companies usually go public to raise money (very few are profitable at IPO). If they are already profitable, what is the main benefit of going public, increased scrutiny, lawsuits, and quarterly expectations?
Liquidity for shareholders. A private company's shares are in most instances essentially worthless or are substantially discounted because the "value" of the shares can't meaningfully be converted into cash (because there is either no market or a very limited market for those shares). A public company's shares are highly liquidable.
