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.
Apparently you did not follow this story well, since in the end the transition did not take place only because of De Jong's decision to stay in Barcelona. Barcelona would have been happy to sell him (and be free of a huge salary obligation) and United to buy him, but De Jong himself didn't want to go to a club that didn't play in the Champions League plus his girlfriend was against leaving Barcelona for England.