Why weren't the Gaza Strip and the West Bank given to Palestinians after the 1948 war?

Primarily because the Arab states had no interest in doing so.

It's important to understand that the Arab states had long harbored the desire to take the area that now constitutes Israel, the West Bank, and Gaza. And that was the desire of previous leaders of Arab states too, including such short-lived ones as Faisal Husayni, who was ousted in 1920 from leading Syria by the French (when the British stopped protecting him, essentially). In 1920, Husayni declared himself king of "Greater Syria", which included much if not all of what is today Israel, and didn't end well for him.

The desires of the Arab states didn't die off when Husayni left, or when the Arab states began to get real independence (some in the 1940s themselves). But they had two issues to contend with in their hopes:

The Zionist Movement's desire, as well as the parallel Palestinian desire, for statehood in the area.
The British controlling the Mandate.

When the British announced they were ceding control of the Mandate, and the UN launched an effort to negotiate a solution (that ended in the two-state partition plan of 1947), Arab leaders participated. While Palestinians officially boycotted the partition committee that visited the area to get first-hand accounts and sights of the area (though they still did meet with committee members, just more quietly), the Arab states tried to wow the committee. The Saudi king, if memory serves, even reportedly presented the committee with gold-inlaid daggers and offered to find one member a wife. The Egyptians went strongly on record as opposing partition, when the final plan was proposed...but not just because they opposed a Jewish state. They also opposed what they suspected as Jordanian plans to annex part of the land, if not all of it (as the Hashemite kingdom saw themselves as the rightful inheritors of that land). And those plans were also pretty accurate: Jordan had even made arrangements with the Zionist movement to annex part of the land, while leaving the rest for a Jewish state. But when war began to approach, the Jordanian king met again secretly with the Zionist movement, saying he could no longer abide by such a plan because it would lead to the war fever on the streets of the Arab world deposing him, and he offered a plan whereby he would annex the area and the Zionist movement would get a measure of autonomy under the kingdom of Jordan, which the Zionist movement rejected.

Egypt itself had designs on the land, as did Syria. They all wanted their own chunks of the territory.

So when the war began in 1948 (after the civil war in 1947, that had all but ended when the Arab states invaded, and Zionists had virtually won the civil war), the Arab states that had promised to coordinate a response didn't. They didn't coordinate well because they had differing aims, and each was looking out as much for its own interests as for the Arab Palestinian interest. There was a lot of mistrust between the states because of this, which helped allow Israel to win.

So knowing that background, it's not hard to see why Arab dictatorships paid lip service to a Palestinian state as a means to an end, and to placate their populations. They wanted the land for themselves, and used the Palestinian population's desire for statehood mostly as a way to justify their grabbing of land. Egypt tried to create a sort of Palestinian government based on the land it took in Gaza, but this was widely seen as a paper government with no real authority, and it ceased to exist long before the 1967 war. Jordan formally annexed the West Bank rather than create a Palestinian state, and said it would be the best holder of the land. It made some initial efforts to integrate the Palestinian population, granting it citizenship and trying to instill a sense of Jordanian national identity in them, though this contentious effort met with mixed success (and hardly anyone recognized their annexation of the West Bank).

The decision, in short, belonged solely to the Arab states in 1948-1967, and they chose to keep the land for themselves. They also chose to create and be the patrons of Palestinian organizations, so they could say they were still working on behalf of Palestinians, who would fight Israel. They diverted attention from their own holdings by pointing to their attempts and planning to destroy Israel eventually and help Palestinians get statehood there...which is unlikely to have arrived, even if they had been successful.