This is because they are cooked in water that is between 65 - 68℃ for 30 minutes, to take advantage of the fact that the temperature at which the yoke hardens (70℃ is lower than the temperature at which the egg white hardens 80℃).
これは卵黄の凝固温度（約70℃）が卵白の凝固温度（約80℃）より低い性質を利用して作られるもので、65～68℃程度の湯に30分程度浸けておくことで、この状態になる。