The position of the Arab League, and the leadership of Arab Palestine, was that Jewish rule over any part of Palestine could not be borne. Israel's victory, sealed by the '49 Armistice, was viewed by all Arab parties as only temporary, and policy was guided by the idea the refugee population would soon be returned in triumph to their land once the Jews were subdued. So assimilation into the populace of a host country seemed neither necessary nor good politics, as it would imply acceptance Jews ruled .
once Arab land.
In the area now referred to as the West Bank, matters were a bit more complicated. Abdullah, the Hashemite ruler of Trans-Jordan despised the Palestinian leader, the Grand Mufti al'Husseini, and was of the opinion his lands ought to include either a port somewhere, or Jerusalem. In '48, Abdullah's forces, British trained and partly British officered, invaded across the river, reaching Jerusalem before the fighting concluded. The Armistice left him in occupation of the West Bank, which held not just its own population, but many refugees from what was now Israeli territory. Abdullah annexed his winnings, which remained under Jordanian control. This was not recognized by most countries, and citizenship in Jordan was not extended to the people there. Egypt did roughly the same with Gaza, as did Lebanon and Syria. The refugees from Palestine were held in a sort of limbo, waiting for Israel's defeat, when they could return.
After Jordan was badly beaten in the '67 war, it's military power was virtually destroyed, and there was a new influx of refugees to Jordan. These began to organize politically and militarily, as the PLO and Fatah, and became a power in Jordan in their own right, one with little respect for Jordanian authority. Once Jordan had rebuilt its armed forces, these attacked the PLO/Fatah, driving them out and into Lebanon, a state also so weak it could not exert authority over these groups. By this time, the thing had gelled: the original wave of refugees had been in camps for a generation, they would not be able to return to lands inn Israel, and were an uncomfortable population to host.
I assure you this account is brutally compressed, and omits much, but covers what I think are the main points bearing on your question.
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