A few thoughts on your question:

"So many" overstates it a bit. There were a handful of "volunteers" who were moles for the USSR. (Not all were Americans, but let's just ignore that.) The Manhattan Project had some 500,000 people work for it over its existence. So that's a very, very low rate of spying, relative to the size of the project. Even if you look at a smaller site, like Los Alamos, which had a few thousand scientists, only a few actual scientists volunteered as spies. It's still a pretty low number. (I am leaving out the other Soviet "spies" who were required to keep the rings running, because they would have been employed anyway and would have been rather useless for the bomb project without the "volunteers." Why the Soviets had spy rings in the USA is a slightly different question; some were ideological/egoist volunteers, like Julius Rosenberg or Harry Gold, but some were trained agents.)

As for the motivations, they varied a bit, but for the wartime period it was mostly an ideological commitment. The Soviet Union was an ally of the United States, and many American Communists believed it was truly the only "honest" power fighting the Nazis, that it was bearing the brunt of the war effort, and that it should have been invited in on the bomb project like the UK was. These aren't totally irrational opinions for the 1940s, though they overlooked the many issues with the Stalinist USSR. Then as now, people saw in international politics much of what they wanted to see.

And many in the scientific community were left-leaning or even Communist. In the 1930s, the Communists seemed to be the ones on the side of the right on many key issues relating to the economy and civil rights. During the Great Depression it was an attractive political position to many who felt that capitalism had shown itself to be dangerous and cruel. This wasn't a fringe position among intellectuals, though many had broken with the CPUSA after the Molotov-Ribbentrop pact. Those who had not broken with it tended to be pretty hardcore Stalinists — the true believers. But the general point here is that many well-educated people had been very pro-Soviet not very long before the war started, and the Soviet Union was an American ally. It was a very different situation than in the 1950s and beyond, where the USSR was explicitly an "enemy."

Lastly, there were some (like Ted Hall) who argued later that a world in which there was only one nuclear power was inherently unstable. Multiple nuclear powers granted deterrence and thus stability. One can argue with that rationale, to be sure, but it's not a crazy idea.