They already have some great strikers like Messi, Neymar, Mbappe to Icardi. Don't they think they will be wasting their money by signing Scamacca from Sassuolo? Or maybe they intend to invest in Scamacca because of his young age (23 Y.O) Even if he moves clubs later, hopefully he doesn't move to PSG, because maybe he won't get game time there. That's why I prefer if he plays for Newcastle
FYI, here's an interesting fact about Scamacca, he hasn't played for Sassuolo again after his first loan to Cremonese. That means almost 5 years Scamacca has not played with Sassuolo.
Icardi is indeed capable enough to be there but if we look at now even though he is a striker position but it is not uncommon for him to be played as a second striker at PSG and maybe they want to start regeneration by buying new, younger players even though it is with his move here doesn't guarantee Scamacca will be in the main squad.