I think the canon reason given for this and other "why didn't the ship's computer just stop them?" situations that it's a privacy violation to just go around scanning people without their permission.
Although they do seem to do a lot of "scanning for life-signs" so who knows?
Yeah, and I've never figured out the security feature that makes scanning for life-signs more effective when you sign a little song to the computer. But sometimes I guess it's just more urgent to know, little life signs, where are you?