The letters O and E became glued together as pronunciation evolved and the letters started to be pronounced as one sound/one syllable instead of two. When the letters are separate, they form 2 sounds. Example:
« coefficient », which will be cut like « co/ef/fi/cient. When the letters are glued together (« œ »), they form a single sound ([e] ou [ɛ]) Examples: “œuf” (= egg).