Because there is no way for them other than trying to continue with their youth especially for young players to be able to play stable when Barcelona needs to rotate players and focus on several competitions at once.
Flick is currently trying harder to prove that he deserves to replace Xavi as coach and of course the initial achievements are important where they are definitely pursuing the target of several trophies at once as a plan for this season, especially for the focus is the Champions League which is indeed in the Xavi era sometimes not too good for them.
Flick knows that young players are future of Barcelona and in their current squad, they don't have enough good experience players to play and rotate in a long season, so he decides to use young players more aggressively. Only few weeks ago, he got a lot of applause for his success with Barcelona and young players by this strategy. After some bad matches recently, he got massive critics, that is unfair for him. Barcelona must accept a fact that they are rebuilding their team, and has to forget an ambitious thinking like they can win La Liga in this season. If they can win it, it's perfect season ending for them but if they can not, it's not terrible ending for them.
Hansi Flick is trying to secure Barcelona's future. Hansi FLick is focusing on Barcelona's young players. Barcelona will become even stronger in the next few seasons if the young players become experienced. Also, Barcelona is currently in a financial crisis. Barcelona does not have the ability to build up a large fund to buy star players. That is why Hansi Flick is trying his best to strengthen Barcelona's current squad. If the young players can play well, Barcelona will no longer need to add new players to the squad. Ansu Fati, Yamal and Torres are all capable of performing excellently. If these players can be properly looked after, they have the potential to become future stars.