Here's why:
* The "o" in "robe" is pronounced with the same sound as the "o" in "go" or "no." This long "o" sound is represented by the vowel digraph "oa" in words like "boat" or "coat."
* The "e" at the end of "robe" is silent. This silent "e" acts as a signal that the preceding vowel ("o") should be pronounced with its long sound.