I think u're confusing habituation with sensory adaptation. Habituation is a learned behavior.. why? Example: Think of a little chick seeing something flying through the sky.. it freaks out thinking it's a hawk or something.. it runs and screams.. then it realizes it's just a plane. Eventually it'll learn to ignore what's flying.. and its alertness will decline.
The fine line between habituation and sensory adaptation is that.. u can purposely bring back your attention and your level of focus for habituations.. but u can't for sensory adaptation.