The answer is some do. And those people tend to get frustrated and quit the field (or, if they are lucky, notice the problem early and never go into fusion at all.) Lidsky is a famous example; Pfirsch and Schmitter in Europe are others. Others bottle up their objections and only let them out as they retire.
The more common coping mechanism is just to assume someone else is going to solve it and not pay too much attention while working on other issues. It helps to focus on physics issues -- which are very interesting, after all -- and tell yourself that this problem is "just engineering" and can't be too hard to solve, in comparison, and that anyway telling yourself it's bad to think about it before the physics is nailed down (which is wrong, but lets you stop worrying that you've wasted your career.)
As for ITER specifically: ITER is costing > $20B for something that could produce maybe 400 MW gross fusion power (and isn't engineered to produce tritium, electrical power, or to survive more that a few weeks at full power). Industrial levels of denial are needed to tell oneself something like that is on a trajectory to relevance.
Perhaps an even better question is: why are people funding DT fusion? For governments, I think it's because in many cases the purpose of government research funding isn't to solve problems, it's to be seen working toward solutions in the distant future (and that perception can be achieved even if there is no actual chance of success, if the public is sufficiently unaware, which it is.) For venture capital I'm not clear what the explanation is. Spinoff technologies like high Tc superconducting magnets, maybe?
Tell me: what exactly would have been worse if the US had never had a manned space program? The whole thing feels like a vanity project, not something that delivered value. It could all have been delayed while launchers got cheaper without loss, as far as I can tell.
Polyimides like kapton are a good example.
Polyimides date back to 1902, but you couldn't buy anything like kapton tape before the space programs created a need for such materials and capital to build out plant for them.
To make a valid spinoff argument, you have to show that the particular technology would not have come about without NASA. That sort of contrafactual, alt-history argument is really hard to do. It's why alt history has such a bad name in history circles -- you can't really show anything. This means all spinoff claims are dubious. Not only are they presented without the necessary evidence, it's difficult to see how the evidence ever could be acquired.
It's often the case that the commercial applications for a technology are not apparent to the researchers doing the research. They're focusing on the science/engineering challenges. Later, sometimes much later, it becomes part of an economically-viable product or service.
Sure, for successful technologies, naysayers were wrong. But almost all technologies (sufficiently finely defined) fail. The naysayers are usually right.
Just like it was for experimental reactors for fission.
Sure it might be unfeasible but you gotta give it a try before judging it a failure
The truth of ITER is that the way the work is shared slow down the project considerably. It would have been much faster as a collaboration between the three or four countries in the world with actual hands on experience on large scale reactor building but ITER is first and foremost a political thing.