That's hardly ever true. In my experience, Hacker News is full of people from the software field; folks whose professions are often unconstrained by physics,[1] and who have a generally optimistic "this can happen" mentality. That's why everyone here believes we'll have fully self-driving cars in 5-6 years.

In the rest of the engineering world, the answer to any question is: it'll be harder than you think it'll be (physics will beat you over the head at every step). You can cross your arms and say "that won't work" and you'll be correct 97% of the time. Fusion is something that some of the smartest people in the world, with enormous amounts of money behind them, have been beating their heads against for the last half century or more. Somebody will figure it out, eventually, but statistically, any given project is very likely not to be the one that does, and it will almost certainly take much longer than the optimists assume it will take.

Did you know that coal plants, which still make up the majority of U.S. energy production, are only about twice as efficient today than they were a century ago?

[1] Although math can be a kick in the ass too if you're working in domains where you can prove there are no polynomial-time algorithms for whatever problem you're working on. E.g. there is a reason people have been banging their heads on the garbage collection problem since the 1950's and it's still an active area of research.