Because he didn't attain it nothing happens instantly unless you Clone (Boruto+ add pulls). That's like me saying why did Sasuke start with ribcage Susa and not V4 Susa.Sasuke attained Perfect V4 Susanoo in Danzo's fight. Why didnt he attain Perfect, then and there?
The databook has set the only condition for it's attainment. Databook = Details Kishi wants to give us.
I'm officially done.