Manhattan and Apollo were both massive engineering efforts; but fundamentally we understood the science behind them. As long as we would be able to solve some fairly clearly stated engineering problems and spend enough money to actual build the solutions, those projects would work.

A priori, it was not obvious that those clearly stated problems had solutions within our grasp (see fusion) but at least we knew what the big picture looks like.

With AI, we don't have that, and never really had that. We've just been gradually making incremental improvements to AI itself, and exponential improvements in the amount of raw compute we can through at it. We know that we are reaching fundamental limits on transistor density so compute power will plateau unless we find a different paradigm for improvement; and those are all currently in the same position as fusion in terms of engineering.