Because you're discounting about 1,900 years of inwardly directed propaganda (for lack of a better term) that kept them unified, prevented their assimilation, and gave them a heritage that they identified with as great, strong, etc.
The Jews of the early 1900s were not a people like the Tamils, who though greater in number were not as persecuted. They were the survivors of 1900 years of migrations and expulsions, who had attritioned by conversions and killings, and reinvented themselves time and again while still holding to the dream and the memory. If you went back to Roman times, not all of modern Israel and Palestine was uniformly Jewish. Not all of it was even ethnically Jewish, as the Idumeans and others had converted, after having once been neighbors and enemies, and there were still significant non-Jewish Canaanite communities.
But once you went from "this is our land we have lived here since time immemorial despite the rise and fall of empires" to "we have been expelled from our homeland, lost our holy sites, and made to feel as unwelcome across the world", you created an "us versus them" situation that was repeatedly enforced across dozens of generations.
Was it ludicrous to give them any credence that they had an inviolable and unbroken claim to a portion of the Levant that had not been majority-Jewish (let alone Jewish-ruled) since before Christianity entered Europe? Yes, of course. But the 1900s was the period of creating ethno-states rather than diverse populations. It was the period where Jewish people, on the heels of experiencing the largest pogroms of their history, unable to migrate in mass numbers to the New World like the Castilians, Irish, Scottish and Germans before them, needed somewhere to go, anywhere at all. Why not their ancient homeland? Why not Jerusalem?
You can understand the perspective of the Jewish Congresses of the time. You can understand why the British and French really didn't give a shit about the region besides protecting the Suez or wanting bragging rights over Jerusalem.