IIRC, after the flood, Noah divided the habitable earth between his sons, Israel being inherited by Shem(source needed). I'd expect that besides Jerusalem Shem would get hold of Mearat Hamachpelah as one of the holiest places. Instead, we see that the descendants of Ham controlled the place.
How come? And why Shem's descendants incl. Avraham, didn't simply claim the ownership of the place?