>Why the fuck does the colonial power dictate who “gets” land?
Because the preWW2 era was a very different time, the Ottoman Empire that previously held the region lost WW1, disintegrated, and the victors were entitled to manage the breakup and created many states.
They thought it would be a good idea for the Jews to have a go at making a state, felt that they needed one to pursue self-determination considering their historical treatment, and had some very wishful ideas about how well the existing population would react or how partition would go.
The British didn't evict Arabs to move Jews in. (Tenant farmer evictions and the associated legal land purchases aside.)