It's because scientists need money to do their work. And their work is to find things we don't know yet, or to check for things we are not sure of : it's a risk

It's because scientists need money to do their work. And their work is to find things we don't know yet, or to check for things we are not sure of : it's a risky and costly business. Because scientists need money to get started, they have to appeal to the interests of those who are paying - i.e the tax payers. That's why the promise always precedes science work. After enough actual work, you'll have a better view of what's ahead. You'll be able to revise the roadmap and to measure the costs, or abandon it ... To sum it up : > ITER will be the first fusion device to produce net energy. You'll never get any funds (nor a high moral among the workers teams) if you frame your research project as "yet another (probably doomed) attempt at nuclear fusion energy".