Why hasn't it generated at least related languages the way Latin has evolved into Italian, Spanish, French, etc?
For two main reasons.
The areas where Greek was primarily spoken were slowly Turkicized and Arabized. The space in which Greek might have developed into new languages had Greeks be assimilated into new groups or expelled.
The Roman Empire continued to exist in the east. The centralized beareaucracy sort of created a standard for Byzantine Greek. Of course the hoi polloi had dialects and such, but everybody had to learn the same language to reach the prestigious administration in Constantinople. Outside of the clergy, no such thing continued to exist in the Latin west. Beaureaucratic government simply collapsed in the West.