Some of the young players decided to leave for themselves. Chelsea can't put a gun to Livramento's head for example, and force him to sign a contract, if he can't see a pathway through Reece James into the first team. There is surely a limit to the amount of academy players a club can reasonably have in the first team. Chelsea already have about 5, of which 3 are regular starters.
Some of them were head-scratchers, I won't lie [Guehi for example, I think is more or less similarly talented as Kounde without the resume to back it up] but in the end, you either send them out on loan and assess their level when they return or you sell them. Most of the players we sold had one thing in common, they didn't want to go out on loan. Those who were happy to go on loan, did [Gallagher, Gilmour and soon Broja].