The Animorphs carefully protect their identities; the Yeerks assume that they're a strike force sent by the Andalites, the alien race that created the transformation technology. To protect themselves and their families from Yeerk reprisals, the Animorphs keep up the facade. Over the run of the series, the pretense becomes harder to maintain; the Yeerks grow more cautious and dedicate more time and resources to capturing the 'Andalite bandits'.