Actually, they didn't. You'd know that if you'd read From Time Immemorial by Joan Peters. A large % of "Palestinians" came from Lebanon, Syria, Iraq, Transjordan, Egypt attracted by Jewish capital and investment during the late Ottoman and mandate periods.

Numbers don't lie and the reports and numbers from British and French mandatory officials are in here: https://medium.com/lessons-from-history/confession-by-the-projection-of-jew-hatred-9d300fbc13ed

The increase in the Arab population during the mandate period was far beyond any possible natural increase due to illegal immigration from the rest of the Middle East.

William Thackeray, Mark Twain and the British consul reported how empty the Ottoman vilayet of Southern Syria (not named Palestine) was in 1857 and 1867.

It was so desolate the Ottoman govt finally allowed non-Muslims to buy land in 1867.

Find me evidence that refutes this instead of yammering on about mythologized fake history.

Then UNRWA defined a refugee as anyone who'd lived in Palestine for less than two years--between June 1, 1946 and May 15, 1948.