Snoop
Mikejk
Think it's to due with them not wanting to go out to others teams and playing in worse facilities and coaches in their eyes. I think there needs to be a balance it's good that they're staying at parent clubs because they're at the best facilities possible for their level but for the sake of their overall development not a lot of players will stay at their parent club for long or even play for the 1st team and need to experience lower leagues before it's too late and they get used to the cushy big club life. Obviously there's a lot more reasons for both strategy's 
There was an article recently about how may of the players in academy teams are just kept on to make up numbers, so the clubs are actively keen not to send them out on loan, unless they are considered a genuine prospect and who would benefit from 1st team football. If the club thinks they are probably going to peak in League One, they will probably keep them around to make up numbers. 
 
Most teams youth teams are full of players to make up the numbers tbf. Jude was the star of our academy sides for years for example