Why were there Nephilim and Giants still on earth after the flood, according to the stories?

FýrdracaDócincel
I consider myself fairly well-versed in Christian mythology but this is something I can't seem to ever find a good answer for.

As far as I'm aware, the so-called "men of old" and descendants of Cain weren't allowed on the Arc. The flood was supposed to have killed them off. But then there were Giants in the promised land when the Israelites first arrived.
Why were there Nephilim and Giants still on earth after the flood, according to the stories?
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