Our forefathers were freakishly rich, but they didn't invest in real estate in the Holy Land for their descendants besides two graveyards (for Sarah in Hebron and Yossef in Shechem).
It appears that they could buy a huge portion of the Land to inherit to Am Israel and significantly ease the future conquest.
EDIT 1: The [innumerous commentaries on the] story of Avraham buying the Cave of the Patriarchs shows the importance of appropriating the land in Israel by our forefathers, with no relations to the recognition by other nations. The Torah would certainly document the buys and we could have much better evidence than only the fact that G-d promised the land to the Israelites.
EDIT 2: Many commentaries say that G-d commanded Avraham to "acquire" the land by walking across, which clearly shows the need to acquire the land anyway.
Why didn't they do that?