Por causa das preocupações morais envolvidas em criar criaturas sencientes
If the Animorphs morphed Leerans once, they should do it again, shouldn't they?
KAA: No, because Leerans are sentient beings, and the Animorphs don't believe they have the right to appropriate the DNA of sentient beings without permission. On Leera they had permission. But now they have no way of asking the Leerans if they mind.
Scholastic.com - "According to K.A." December 1998