I mean, to be fair, so is slavery.

I think it's important to acknowledge that conscription is a violation of human rights, and absolutely a violation of human autonomy and dignity.

The reason it exists is because historically, the primary source of military power has been the number of armed humans you can put on the battlefield. That's much less true now, but even in WWII the technology wasn't yet up to that point.

And while the US did not end up being materially at risk in WWII (aside from some very small exceptions like Pearl Harbor), that was not a guarantee going in. The Nazis were hellbent on wiping out all opposition to them, and the fear that, if Europe was lost, they would cross the ocean to attack us was not at all crazy. Furthermore, our allies absolutely were under existential threat—and in such a situation, it's frankly irresponsible of a nation not to use conscription if that's actually likely to make a difference.

Either saying "conscription is slavery, therefore it is never justifiable" or "conscription is nothing like slavery, soldiers get treated well" ignores enough of the truth that they're misleading at best. Sometimes you really do have to deal with nuance.