Why? Well because it is kind of different either you will take a kid, put your arm around him, take him to senior side, give playing time and help him to develop into starter (my fav example is Thomas Müller who was promoted from essentially 1st game of van Gaal reign, and has gone from heights to heights ever since) and a star....
OR you will "play" let's say Mbappe, who might have been 19 when he joined PSG (so by "playing young" criteria, any manager who plays this 200 mil superstar is visionary and "trusting youth"). I mean you have to be extremely stupid not to do it... Of course Mbappe is an extreme example with his profile / skills / price. Most are more tricky.
OR it is 3rd kind of situation, where you come to club, someone else has taken leap of faith already, player had already proven that he is able to play on this level, and THEN when he has fought himself a place in squad you just continue playing him (say Kane's and Bentaleb's position).
It is extremely obvious to me that the 1st option of those 3 is most complicated and valuable. 2nd - everyone can do. 3rd is a lot-lot easier than 1st.
P.S - well, yea, years ago Poch really used to play youngsters. I agree with you. But this is distant memory. Look at the guy's last seasons and what is the situation with say KWP, Parrott and Skipp. I'd say it was borderline criminal mismanagement of young talent just having them rot on the bench. But we'll see.
Anyways, facts don't lie and when Poch left the helm, we were one of the oldest sides in EPL...