Even if we could be bothered, I think we've lost the political technology to organize large government projects and complete them on a sane budget/timetable.

The budget plan for the James Webb Space Telescope was $500m with completion in 2007. We're now at $9.6b with completion predicted in 2021. Some of the cost overruns and delays were caused by the unexpected complexity of fabricating the mirrors and assembling them in space, but that underscores my point. The Apollo mission was orders of magnitude more ambitious at the time.

I'm not pointing any fingers because the causes are complex. NASA is underfunded and relies on unreliable funding from Congress, which itself is a source of inefficiency. Defense/aerospace contractors are greedy, sure, but there's a much bigger pie to be had if past successes encouraged even larger-scale projects like moon colonization they could bid for.

I don't know what's gone wrong, but we are failing on these big projects as a species for some reason.

Yes, that does seem to be true. Though I wonder if the incredible amazingness of the first moon trip together with a sense of competition with USSR actually gave workers throughout the project more personal motivation to put in real effort instead of just being minimum-effort drones in a bureaucracy that doesn't matter to them. Doing it all again will never be the same. Even going to Mars won't be such a massive leap.