It's because both value the trait of a player being proven over a player being good enough - if a player is proven at the right level then it goes without saying (or should) that they are good enough so you get both requisite traits and less of the gamble. At this level we simply can't afford to allow one player to learn from their mistakes because those mistakes could place our future as a top-level club at jeopardy. When the difference between winning the league and coming second can be +6GD (as we have witnessed) and the difference between coming first and second can be the catalyst in a club losing confidence and sliding down the table, possibly not to return easily if at all, then you have to treat every goal scored, let alone every point won, as a potential make-or-break moment in the club's future.
At least, that is why some managers/clubs simply can't afford to play youths. Not sure if I agree, and I'm certainly not sure that I'm happy for it, but that's what causes the reluctance.