They do because the other part of the equation is that they need to keep spending a lot of money to build out infrastructure faster than their two most significant competitors, one of which is public and wildly profitable (for non-AI reasons) and another has already filed for IPO. So it puts them at a disadvantage to walk away from a massive cash infusion.
I think the answer is simply that private money is cheaper than public money for these companies right now.
OpenAI hasn’t had any problems getting impressive amounts of funding. So why ipo?