From the start, the Animorphs have made this self-imposed rule(s):
1.
Never morph Sentient creatures without permission.2. Never write off an innocent life.
3. Fight the enemy, don't become them.
Although the Animorphs find it impractical to follow this rule, they still do, because they also believe that.
"We fight Yeerks, we don't become them".
Controlling other sentient beings' bodies is wrong. It is what a Yeerk would do.
Plus, Jake couldn't and wouldn't go... He was the one irreplaceable person to infiltrate the pool ship. He was the leader of the only Earth Resistance faction. Only he had the most influence to persuade the Andalites to give them the morphing cube. Plus, who would give orders in the battle inside the ship? Rachel?