Because those saying that believe it to be enough time for them to retire or move on to something else.
Well that estimate is better then what my 1990s plasma physics prof said - Fusion power is 20 years away! And always will be
They don’t want there to be unlimited (not really) energy. Because it would be cheap and the powers that be wouldn’t be in a position to control it. Too much money at stake to allow it to be fully developed. As long as they can keep the knowledge hidden from the public, it will stay 10 years away.......