Because it is so easy to see in the scriptures.
Sinless?
34 Awake to righteousness, and
sin not; for some have not the knowledge of God: I speak this to your shame.
1 Cor. 15:34
Paul says if we are sinning we don't have the knowledge of God.
Be perfect?
48 Be ye therefore
perfect, even as your Father which is in heaven is perfect. Matt. 5:48
Christ (the head of the Church) says as plain as day his disciples are to be perfect as the Father is perfect.
So not only is sinless perfection scriptural it is
ESSENTIAL. It is not presented as an option but as the only true standard given the disciple.
Everything else is delusion to try to keep you from giving this your attention.