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?