It's really not just down to hoarding talents, they excel at selling all kinds of players. I'd imagine there's some fuckery going on ie account cooking because I really can't see how one club so clearly outperforms everyone in that aspect. Maybe they allow clubs to draw sales out until infinity, maybe they pay parts of the fee back in blood diamonds but it's outrageous how well they're doing in that aspect.
There are plenty of well known examples so I'll pick another one in Kurt Zouma. Was never a genuine top class prospect but a decently talented youngster who they bought for 14m in 2014. He then has mostly middling to mediocre performances for them then tears his ACL in 2016. Somehow Chelsea manage to loan him just half a year after returning to Stoke for 8m (without a buying option) who get relegated, then they secure another 8m loan to Everton the year after. After returning from Everton Chelsea keep him on their books because they're hit with a transfer ban, then they sell him for 35m to West Ham last summer. No other club manages to get even close to 50m+ for a frankly shit defender bought for 14m with a torn ACL, no one.