A PhD comes as a student with F1 student visa that expires the day of their graduation.
O1 is unlikely to be granted to a student who has not graduated yet. What are they going to show for evidence? Manuscripts in preparation? Or class grades?
I guess they wouldn't have much to show for evidence. Which is exactly why they would be correctly classified as not being a specialist, and therefore undeserving on an O-1 visa.
These visas are not meant to allow company to hire underpaid employees that quite literally just graduated.
A company conducts a technical interview to assess a candidate. Their public record is not their only criterion of hiring. USCIS relies exclusively on public record (and maybe recommendation letters)
