There's no reason to think that megastructures are only achievable through slavery, but I think it's fair to say that it's a lot cheaper if you're willing to disregard the humanity of others and abuse them until you get what you want. The alternative is that you pay workers what they're worth and use enough of them that they aren't being overworked, but that eats into profits.

I think we still take plenty of risks, still do big things, and still enslave and abuse a lot of workers. It's increasingly seen for the evil it is, but that hasn't stopped it from happening. I think the biggest reason you don't see as many massive projects these days is because we've already got a ton of infrastructure in place, major technological advances are getting harder to come by as we've covered a lot of the "easy" stuff already, and the emphasis on short term/immediate profits.

When we suddenly need a massive structure to house a major sporting event like the world cup or Olympics where a small number of people are basically certain to make a fortune you'll find we're still perfectly willing to construct it on the backs and corpses of forced labor and migrant workers suffering abuse, only to abandon it afterwards until it's time to build a new one somewhere else.

Advanced aliens (and we 'almost') will have robots for that. And they would also have less resource issues than us, so, they would have trillions of them.

Once I had well paid job at American company in Germany which paid nice salary. Consider Apple’s iPhone. You have it, I have it and it’s a technical mega project. When you do a teardown, there are hundreds of different components. There was dozen engineers working on the smallest part. Hundreds if not more on the processor. Thousands on manufacturing, logistics and retail. These people don’t dig dirt all day long. But trust me, design, build all the parts on time, assembly and ship the phones to stores on time is absolutely a mega project. But outsiders don’t see this. Imho that’s real large scale global project.

On a species level, you can imagine an aggression/cooperation "species personality" axis. Humans are in the middle, with chimps more on the aggressive side, and bonobos to the cooperation side.

Being in the middle, humans have a bit of a split personality. We cooperate on a large scale during warfare. But consider the Cold War. Both the US and the USSR were continent-spanning countries with multiple ethnicities. I would argue that cooperation on that scale just isn't that different from cooperation on a planet-wide scale. A species that's capable of one is very likely to be capable of the other. That's part of why I'm not terminally pessimistic about humanity, or starfaring species more generally.

I don't think we can rule out starfaring for a species that's a little more bonobo-like, and defaults to a post-national outlook.

The Space Race was partly a Cold War propaganda program but it diverged almost completely from the ICBM programs. ICBMs have to be solid fueled to minimize launch time. But manned orbital launchers have to be liquid fueled (for the core) for efficiency and safety.

The world wars drove more technical progress than the world has ever seen, before or since. (Making your iPhone better at doing the same thing worse and slower so the end result comes out basically the same isn’t “progress”.)

Indeed. Some of those bacteria would love to consume your megproject. As you soon as you lose power to resist, they get a banquet of a lifetime!