If AI business is greatest business ever, why are they doing IPO?

I understand need for more money and more compute. But if they are so advance and great. Why are their own private investors and private share holders letting this multiple trillion trillion level. Sure they could raise until Z letter in private rounds, stay private like some companies be able to for for their entire time? Why sell ownership of the greatest business ever created?

Standard Oil never went public, they had more than 10% of the GDP at top and true, self advancing AI would worth multiple times higher? There is not even monopoly breakdown risk because there are already multiple competitors.

Or simply put, they don't believe the current valuation justifies significant future growth. They need to exit their position, especially given the risk of volatility around the current valuation?