Could be a number of things as to why they keep it under wraps. 1. Keep the attention off the youngsters, keep them out of the limelight and more grounded. If they're not personalities they're less likely to get in trouble in public. 2. Keep opposition clubs from having too much data and knowledge of them, especially the likes of German clubs who like poaching young players. I know they can send scouts but it makes it as hard as possible. 3. Stop the media and fan clamour for people to be called up like they do with Foden, and as happened with Sancho and Diaz who became known, who became in demand and then got coaxed out. If they'd stayed behind closed doors and nobody saw anything of them they'd have had less bargaining power and may not have been enticed to move so young.