That is a widespread gap in knowledge of the process of vipassana caused by the way the suttas are oriented towards arahants. Looking at the Anapanasati sutta, it can be seen that impermanence doesn’t figure until the fourth tetrad:
" He trains himself, ‘I will breathe in focusing on inconstancy.’ He trains himself, ‘I will breathe out focusing on inconstancy.’ —-MN 118
While the preceding third tetrad is a precursor exercise (for Satipatthana) for removing unwanted mind states:
" He trains himself, ‘I will breathe in sensitive to the mind.’ He trains himself, ‘I will breathe out sensitive to the mind.’  He trains himself, ‘I will breathe in satisfying the mind.’ He trains himself, ‘I will breathe out satisfying the mind.’  He trains himself, ‘I will breathe in steadying the mind.’ He trains himself, ‘I will breathe out steadying the mind.’
Now the account of Satipatthana contained in the Anapanasati sutta expands on the exercises outlined in the tetrads and introduces a new theme, “putting aside greed and distress with reference to the world,” which means subduing anger and desire, which is stressed from the first to the last verses. Again impermanence doesn’t appear until the fourth verse.
This shows that right effort is a ubiquitous factor throughout the Anapanasati and particularly Satipatthana suttas, while the insight factor of reflecting on impermanence only appears in the fourth stage.
Throughout MN 117 right effort constitutes the active factor for change in right view, which is the essential process of the path, and knowledge of impermanence is subservient to that process:
"One makes an effort for the abandoning of wrong view & for entering into right view: This is one’s right effort. One is mindful to abandon wrong view & to enter & remain in right view: This is one’s right mindfulness. Thus these three qualities — right view, right effort, & right mindfulness — run & circle around right view.”