Sforno on Genesis 6:4/ספורנו על בראשית ו׳:ד׳

1הנפילים היו בארץ בימים ההם, these giants were not about until the last 120 years which G’d had assigned man as a grace period within which to do teshuvah וגם אחר כן, as well as thereafter, i.e. they did not repent and mend their ways at all.