The word “yolk” was derived from an old English word geol(o)ca, literally “the yellow part,” from the word geolu meaning “yellow.” Thus, an egg’s contents are the egg white and egg yellow. – WTF Fun Facts
Source: yolk | Origin and meaning of yolk by Online Etymology Dictionary (etymonline.com)
It just is
Which has a German wordstem, like a lot of English words. In Germany we still call it ‘Eigelb’ and ‘Eiweiss’, which is eggyellow and eggwhite.