Because there's literally no one going for him at the moment except Liverpool( Chelsea , Arsenal and Mancity are all settled) who are on spending spree after investing Coutinho's money on ETH at $600 some years ago. 

Seriously, if Newcastle can't find anyone, they won't allow Isak leave.  He's still under contract and must play if they can't find a solution and if Liverpool aren't willing to pay the €150m asking price.

Newcastle saw J. Pedro, Ekitike and now Trafford escaped through their nose and now Isak telling them he wants to leave - this is where i love Daniel Levy approach to situation like this.