Why don’t we have fusion power plants yet? What is the central challenge to producing fusion energy on Earth?

“Terrestrial fusion requires plasma to be confined and maintained at high enough temperature for long enough time—the Hot Enough/Long Enough (HE/LE) requirement. “Hot Enough” means ≥100 million °C, which is achievable. “Long Enough” is the challenge, because plasma loses heat rapidly to its surroundings. A commercial fusion machine must minimize that loss to achieve net energy gain.”