https://www.motorsportweek.com/2023/02/10/horner-suggests-alternative-method-for-andretti-to-join-f1/Christian Horner says that Andretti should buy an existing F1 team instead of building one from scratch.
Except Andretti already did that, working with both Sauber and Alpha Tauri. (I have no idea what sort of relationship Horner has with Alpha Tauri, though surely he would have been aware of the negotiations?) The only team that's really 'available' is Williams, and I would assume that Andretti has talked with them too. Maybe Haas?
Obviously buying an F1 team has advantages to hit the ground running, so to speak, rather than spending a few years losing while learning the ropes. I do suspect that Andretti is keen on having it run as an American operation (whether that's a good idea or not remains to be seen), so buying Williams, or any European based team, might end up losing jobs (which Horner is against) from an existing team unless they're willing to relocate to America. Haas would be a better fit, they're using the American/European satellite model that Andretti is hoping to use, and I'm sure it would be easier to convince personnel to move from North Carolina to Indiana while keeping the Banbury site/personnel active.
Knowing that Andretti wants into F1, ANY team looking to sell would have contacted him. If an F1 team was available to Andretti, on Andretti's terms, I can't imagine he'd have turned the deal down.