Because their logic processors aren't sufficient, for instance, if Finn Lewis gave them an order to protect Kay Lee's safety, and if Kay Lee was in danger, they would immediately eliminate the source of the threat. This would quickly expose their identity.
Unless Finn Lewis equipped them with a t2500 type command, but a standalone t2500 type costs a full 50,000 exchange points, which is also a burden for Finn Lewis. Moreover, its combat ability is not remarkable. As for another t2000, with a stronger body than the t800, and a logic processor that is no less than the t2500, but the exchange price is also daunting for Finn Lewis, as high as 200,000 exchange points, that's too much for him to bear.
