Although he is the second choice in Barcelona squad, he can play with quality players. He also seems suitable with Xavi tactics, so he feels comfortable staying in Barcelona. While if he moves to Man United or Chelsea, he will be the main player but may not be suitable with the tactics of the coach on these clubs. He also may feel uncomfortable with a bad situation the new clubs, especially Man United or Chelsea. These clubs are in a very bad situation this season.
It seems that you really don't understand the situation or really don't follow the player's transfer progress. I'm not a Manchester United supporter, but the rumors regarding Frenkie De Jong's move to Old Trafford are the longest in the 2022 summer transfer window.
De Jong is a player that Erik ten Hag wants so much that throughout the summer transfer window of 2022 Manchester United made several proposals to Barcelona although Barcelona also refused several times. That's a sign that Erik ten Hag really likes the former Ajax Amsterdam midfielder. It is very unlikely that De Jong would not like the tactics of the Manchester United coach if he played in England with Manchester United.
In the transfer window last summer, Manchester United were desperate to sign De Jong from Barcelona.
even the red devils, reportedly have reached an agreement with barcelona regarding the transfer of de jong. however, de Jong's transfer failed to materialize because the player was reluctant to join the Man United squad. there were actually a lot of rumors about a move for de jong in the last transfer window.
but clearly, no agreement has been reached so far. de jong himself is still needed by Xavi to this day to stay in his plans, however, if in the next transfer window barcelona can get Benardo silva from man city, there is no reason for Xavi to keep de jong at camp nou.