Because current history tells us that after WWI and the defeat of the Ottoman Turks after 400 years, there was a British Mandate of the geographical area of Palestine, and after a quarter of century, unable to deal with the problems, the British turned it over to the United Nations who came up with a partition resolution to give two stateless people the opportunity to build two independent nations. Jews accepted and Arabs chose war and lost. Was there any injustice? Yes, a lot of Arabs got a raw deal because of the decisions of their leaders. This happens in every war and every transition of power. We know what would have happened if Israel had lost the war.
The Jews of Israel went on to build a viable nation at the forefront of science and technology, and Palestinian Arabs, in 2024, still do not have full membership in the United Nations because they do not meet the requirements for statehood.