That's sort of a misconception actually. One of the Zionist slogans was something like "A land without a people for a people without a land" and this was based on the perception that Palestine, or parts of its agricultural land, were not heavily populated but this was not necessarily true, or at least not as true as was believed by Zionists, and was based on some faulty info. There were images of Palestine that circulated showing ghost towns devoid of any inhabitants but the reason for this was because those towns were basically seasonal towns and were left empty for part of the year but inhabited and farmed for the rest. But the perception was too attractive for those who wanted to see a state for the Jews so it persisted. Its true they only bought part of the land but they did also eventually expel Palestinians and take land from them by force as well.