Locke could obtain the hidden evolution route of the Battle Beast through the system, thus turning it into a treasure.
He could turn the originally weak and useless Battle Beast into one with astonishing combat strength.
For example, the Swamp Hydra or the Divine Light Dragon.
It was only when the Battle Beast egg or the baby was discovered by Locke that he could successfully sign the contract and evolve through the hidden evolution route.
The same could not be said for a Battle Beast in its growth or maturity stage.
Battle Beasts were the same as humans. They would have their own consciousness when they grew up.
If one forcefully signed a contract with a Battle Beast, not only would there be a high chance of failure, there might even be a situation of betrayal or even a backlash.
This was something that Locke absolutely could not allow.