I think you'd have to qualify the "we" here, and even within the United States, education on this varies dramatically by classroom and location. But if you're asking why the "orthodox" narrative about the use of the bombs is prominent in the United States despite most academic historians being well aware of its inaccuracies and complications — it's because the atomic bomb is not really being taught as "true history" (an account of what happened, good and bad, warts and all, etc. etc.), but it is used as a stand-in for teaching about a form of "ends justify the means" logic. The atomic bomb decision becomes a way to try and create a moral reasoning that squares the ideals of the United States (which don't include the mass slaughter of civilians) with the reality of its actions (which have at times included the mass slaughter of civilians, among other activities that conflict with its ideals, like deposing democratically-elected politicians in foreign nations and replacing them with dictatorships, and things like that). If you accept the framing of the orthodox version of the bombing ("either we dropped the atomic bomb on these cities, or even more people would die") it becomes a template for excusing all sorts of other unpleasant activities ("either we depose these leaders, or the Communists would take over, and then lots more people would die").

The fact that many teachers (and journalists, and certainly politicians) do not even know about the complexities of the atomic bomb's history, and the historical problems of interpretation and so on, doesn't help any of this. It becomes a self-reinforcing issue even if people have good intentions about it. I am not saying that everyone teaching this history this way believes this is what they are doing, or is self-consciously aware that this is how it has become deployed. But this is my sense of how it is deployed in the American context, and what this deployment ultimately does for the sensibility of Americans.

The answer shifts if you look at other countries, interestingly. The Japanese have long taught that the bombs were necessary not because it is in favor of the above narrative, but because it allows them to shift the story to being about militarism and war in general ("if you go down a militarist path, you will end up being destroyed"). Different historical stories have different consequences, and different political cultures make different uses of them, and they thrive in various political contexts better than others. This can be said of the opposite, counteracting narratives as well — e.g., the narrative that the bombs weren't necessary at all, that they were only used to scare the Soviets, etc., is one that thrives in certain (usually left-leaning) political contexts that like it because it paints a very different story about the United States.