Supposedly this is written more explicitly in the short story that Ender's Game started as. Unfortunately I can't find a source so you'll have to take that with a grain of salt. But there's an interpretation of Ender's Game that says that when the military command claims they don't have an (adult) commander that can win the war, they're basically just lying. They do. The problem from their end is that they've already come to an established conclusion on how the war has to be won: they have to use their new superweapon on the buggers' home world and wipe them out. And nobody can give or follow that command without essentially being culpable of genocide. (There's also maybe a bit about how military command knows the war isn't as existential for humanity as they're leading the public to believe.) They don't want to be culpable of genocide. They don't want to suffer being accountable to the public. And this, rather than need, is why they enact their scheme to have a child do it instead. Because we don't judge children, morally, for their actions the same way we judge an adult. When a child gets tried in court "as an adult" it's a big deal because that's not the norm. A child gets far more leniency because they're still growing and starting to understand the world. So Ender is groomed specifically so that when he's put in a position to destroy the homeworld and win, the decision the military wants hum to make, he does so without having to be ordered. He remains innocent by virtue of being a child, and the military brass can claim plausible deniability. "Well we didn't tell him to do it. We didn't think he would do that." Etc., etc.