Maybe Nico was offered a much higher salary which Barcelona can't afford to pay him currently or could be anything, no one knows his reasons. If not for form Rashford is a much better option than Nico, he scores a lot of goals and also a very skillful player. I also have same feeling that Rashford will do well at Barcelona because he plays a much similar style to them and will enjoy all the spaces they create.
In my opinion, I think Athletic Bilbao never wanted Nico Williams to join Barcelona in the first place; that's why they made it difficult for Barcelona to sign Nico Williams, and when Athletic Bilbao found out that Barcelona couldn't pay Nico Williams's transfer fee at once, they quickly rushed to give Nico Williams a lucrative 10-year deal with a new release clause just for Barcelona not to sign Nico Williams.
Bilbao has always had a unique policy because they always recruit or retain players from the Basque region, so it is only natural that Nico is now an asset not only for the club but also for the national team. They will do their best to retain him because they sometimes do not care about money but always uphold their pride.
We can see how the fans express their objections when Nico Williams, who is often linked with Barcelona, is even having murals dedicated to him removed as a form of protest. Therefore, it will be difficult, regardless of the price, to know what will ultimately happen for Nico Williams, especially since he is already tied to a very long-term contract. It would be better for clubs interested in Bilbao players to seek other options, as negotiating with them will be challenging due to their strong ideology, which they consistently uphold.