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".