Sancho's rumoured 25 million euros fee isn't that huge honestly. The real problem is his salary... Why?
Because he makes 7.8 million pounds (~9.2 million euros) a year at Manchester United. Chelsea didn't even pay his full salary during their loan contract.
Is Napoli or Juventus keen on paying this much money per year to him?

I don't know whether he is keen on taking a pay cut.
Sancho is reluctant to accept a pay cut, which makes Chelsea reluctant to make his status permanent from Manchester United. With inconsistent performance, I think Napoli or Juventus will also think twice about meeting Sancho salary demands. Seeing Sancho bargaining power in negotiations, I think he will eventually be willing to accept a pay cut in order to get a new club.
Manchester United wants to offload a player who has become a burden on the club. Ruben Amorim is hoping to profit from Sancho sale, which can be used to recruit better new players. Logically, if Chelsea, with their more stable finances, are unwilling to meet Sancho salary demands, Napoli or Juventus are likely to take the same stance. Should a deal not materialize, Sancho could find himself on loan once more next season.