> Caring about every penny you spend is definitely not how you want to run a company doing groundbreaking work, but there's definitely a middle ground.
Private business allocates funds toward profit. Apparently, investors don't see much potential return on additional funding compared to, e.g., electric cars, phones, online retail, etc. An investment's value can be calculated, in one sense, as risk x potential profit. Given the latter is very high, investors arguably must perceive the former as very poor, though possibly they perceive that the companies don't need more funding at their current stages (i.e., it's marginal return on additional funding that matters for investors, not total return on all funding).
The people of a country, through government, can allocate funds toward work based on importance and regardless of profit. Such work, such as basic science, is where the market fails. (Probably much of the basic science behind fusion was already funded this way.) Per the OP, we've allocated $22 billion toward ITER. So fusion is pretty well funded.