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.
So indeed the rotation of players by taking risks like this is one way for Flick to test the readiness and mentality of the players and the results I think on several occasions of course we see that it is a very good thing.
Flick knows what he wants from his young players and how to use them, teach them and help them to improve their career as well as have more and better contributions for Barcelona. He is doing his great job for Barcelona not only in this season but in future.