It wasn't their land. They gained it through violence and coercion. Also, it has caused tons of conflict and death since it's creation.
Imagine you live in Maine, and all of a sudden a ton of Rwandans move in and bribe officials, use their money and international influence to get credibility for the idea that Maine was always a place for Rwandans. Then, they start blowing up hotels and committing other terrorist acts. Then, the UN says that Maine is a country for Rwandans as well, and gives them 78% of Maine for their own nation. People in Maine don't agree, and are immediately subject to extreme violence and attempts at ethnic cleansing. Rwandans establish a country of Rwanda and the most powerful nation on the planet immediately recognizes it as a legitimate country and government. Does that sound even remotely reasonable to you?