Because you have to manage bringing in a bunch of young players.
You can just tell Willian to do what he does, but when you tell a 21 year old Havertz to "Make magic" it doesnt happen the same way.
Havertz looks in over his head whenever we play any semi decent side. Its the coaching too. You have to coach a young player. You cant just give a 21 year old a free role and tell him to make magic. Its setting him up for failure.