Former fusion startup founder here. This is a very good point. My startup took 6 years to fail and much of our competition at the time is still at it 20 years later.
There are a number of things which are also impedance mismatches between the typical SV startup mode and hard startups. One of these is the failure path. When an adtech startup fails, the founders just go get well-paying jobs at a FAANG, or at least that what the perception appears to be. When your fusion startup goes bust, you... get evicted and live in your car for a while. There's no on-ramp for failed founders back into the industry. Heck, if its a hard startup, there may be no industry yet to go back to. So, you have to go find something else to do to pay the bills. The personal cost of failure can be much higher than typical startups. "Our incredible journey" for hard startups is more like, "Well, none of us died".
Another thing which doesn't mix well between the typical SV expectations and hard startups is that all the things which VCs think are advantages to being located in SV are generally disadvantages to people working on hard problems. Doing web software with ads? Lots of people to recruit in the Valley. Doing nuclear fusion... is there anyone doing nuclear fusion in the Valley? How do they afford to live when competing with all that cheap ad money? Perhaps I should locate on Madison Avenue, it has the same "advantages".
Then there is the typical way that VCs filter, starting with step one, "Know someone who can introduce you". That's a lot easier when you are in the same location, working in similar industries. It's a poor filter when most of the people working on a problem live far away, doing stuff very different from everyone you know. There are far fewer serial entrepreneurs when the failure time is measured in decades, so networking is more difficult.
I'm happy to see VCs try to return to working on hard problems. The Valley used to be good at that, before they became a center of the advertising industry. But Valley VCs need to read their history. In the late 1950s/early 1960s, New York was the center of finance and advertising. The idea that somewhere out in the wilds of California would become a center of a new industry with its own finance ecosystem was crazy. But everyone in the early Valley knew what those East Coast people were like. The Valley is now the new East Coast people, the new Madison Avenue. Doing a hard startup in the Valley now is like trying to start Intel in 1960 New York. Hard startups are hard enough, without trying to do that.