There's a lot more nuance and potential legal issues involved though. There certainly are some things involving the transfers of U18's we cannot replicate in FM. Don't conflate this with "everything works fine right now" but there's an awful lot elements involved with U18 transfers that probably wouldn't stand up in a court if there were two competing clubs involved or a dispute arose. This has happened a number of times historically.
Can Endrick legally sign a contract to play for a Spanish team in 2 years time at 16? I genuinely don't know the answer, but if everyone agrees to things in principle, the clubs are happy with the money, other invested parties are happy with the money, and no one rocks the boat it probably goes through fine.
It's an area we're working on the balance of moving forward - like many areas of the game such as the ME. More save specific examples where players are on senior/professional/fixed term contracts and you feel they ought to negotiate but don't will be beneficial to try and reach that moving forward.