Why is cheilanthes not spelled keilanthes?

Greek has a letter, chi, with no English equivalent. When Greek words came into Latin the convention was to write it ch, and English inherited the convention along with the words. So chemistry, chorus, chasm and cheilanthes all begin with a hard k that is spelled ch. Nothing about the sound will ever tell you this. It is a piece of history sitting in the middle of a plant name.

A c that has to be read as k, because the letter behind it is a Greek kappa.