Hard Startups
The problem with startups building hard tech is it takes a long time to fail. Failures that take decades doesn't work in the startup model. Image working on a fusion reactor. It might take 25 years to fail. The failure may not be that your idea doesn't work, but because some other kind of reactor comes along that is a better business (ie, cheaper). At this point, you are now a world class expert at a technology that is not economically viable. But, you can't transfer that knowledge, or at least most of it. You are also now 50 years old, give or take. You don't have 25 years to work on the next hard problem. You certainly don't have time to fail again. Back in the day when lots of people worked on hard technology, they were paid well to work on the problem; their life time earnings did not depend on creating a viable business. Because of that, they usually earned a pension, too. Because if you failed, you could retire comfortably, or, at least have enough to choose a second career that didn't pay as well. That model doesn't work for startups.