Well perhaps these youth players aren't considered good enough either. I'm sure our scouts and coaches are watching them regularly in training and on loan and know more about what it takes than you or I. Not being funny, but it's also harder for youth players to break into our team then yours. That's the price of having an owner who demands instant success (an argument for another day).
KDB was not better than Oscar or Willian, I can tell you that as fact having watched all 3 a lot. KDB had promise but didn't want to put in the work required to make it at Chelsea and with Jose. He'll have a good career as some clubs will be happy to have him as a luxury player. He wasn't good enough to be a luxury player at Chelsea. Look at how hard Hazard (a far superior player) works.
Don't cry for KDB, we picked him up from a small club in Belgium and he has since become an established international, played in a world cup and got a decent move to the 2nd best team in Germany. Not too shabby. In fact it's better then any Spurs trained youngster has achieved of late.