Why do some say that the land which the Palestinians claim was never really theirs to begin with ?
The some are Zionists, and their supporters.
They did that to justify the past atrocities like the Nakba/Catastrophe committed against the Palestinian people, their ethnic cleansing, and dispossession from their lands, and properties.
They do that now to justify their apartheid system, illegal settlements expansion, Palestinian land theft, and forcing Palestinians into more Bantustans.