Noach worked on that boat for many many years. Over a hundered!
Since Noach and those before him had invented practical tools made of
metal, why should it have taken him so long? HaShem wanted it this way so
folks would come and ask: Why are you, in the middle of dry-land, working
on a large boat? Noach could then answer that because of the destructive
ways of the generation, HaShem was bringing a flood.
Tshuvah was not only available, but the object!