It's insane that they aren't public yet. Their investors must be pressuring them like crazy to IPO.
Stripe has been doing annual tender offers. Their stance on not being public yet is that they don't need to be, as an IPO is mainly a way to raise money. As an ex-Stripe, I understand the sentiment, and the tender offers are a nice middle ground for now, but I still would like to see them go public eventually.
Raising money as a private entity is trivial these days if you're in the league that Stripe is. See: the comical AI private funding levels.