Genesis 6:5-14 is about the wicked generation God wiped from the earth and one man who was righteous was rescued. When we rebel against God's design it takes time but there are consequences. Scholars have estimated there could have been 5 to10 billion people living on the earth before the flood.
The average lifespan back then was 816 years. Adam died 126 years before Noah was born. The flood occurred 726 years after Adam died. Noah was 600 years old when the flood came. He lived another 350 years after the flood and died at age 950.
It wasn't just a few years of the people being wicked for God to cause a flood. Years of warnings for them to turn from their wicked ways. It took Noah about a hundred years to build the arch.
Before the flood every intention of thoughts of mens hearts was only evil continually. Not back and forth, constant evil, EVERY thought on their hearts and in their minds.
Noah was a preacher of righteousness, he found favor in God and did all God commanded him to do. God's choice to rescue one man's family out of the multitudes, reveals just how personal Jehovah is. A singular action affirming that our Sovereign Judge destroys no man without cause.
God looks down with favor upon those who look up with faith. God is patient and he waits and waits always inviting us to turn. God is patient beyond our understanding, not one of us would be ever so patient the way He is.
Noah's faith saved him from destruction; that very same thing can save us too.