The current Barca (sadly) don't have the patience for it.
Yeah, I mean it's of understandable that they don't want to make too many experiments at the level at which they play, but even PSG or Bayern take their time to promote some potential and that seems to pay off for them. It's a bit sad to see that the trend at Barca currently is heading to bigger names with experience who are peaking. For me personally it's definitely more exciting to bring in some new guy, then make him a worldbeater instead of buying all these players that everyone knows inside out already. Barca is not THAT much in trouble in the midfield, that they'd always need immediately succesful players.
Hell, even fucking Madrid has been promoting a fair share of younger unproven players in the past few years, don't let them have all the fun.