Answer:
They were in more natural poses so arms are more likely to break
Step-by-step explanation:
Even without the provided examples, the logical explanation is that Greek statues were in natural poses, so their arms were not glued to their body. They could then more easily break, as they did not connect to the rest of the statue so much, but had only a small part of the connection.
Statues coming from the cultures like Sumerian and Egyptian (first and second picture) were stiffer, their arms often right beside their bodies or on their chest, their necks thicker. This ensured that they were part of the “main” body of the statue, glued to the massive block and that they could not be broken that easily.
Greek statues were made in many details and natural poses. They would more likely have the hands above their heads, their neck would be tinner, they would be performing some action, etc. Because of this, they were more fragile and had more places where they could be broken. For example, the famous statue Winged Victory of Samothrace (third picture) has many details, as well as fragile wings, arms, and neck that are all briefly connected to the body with the thin part of the material. This makes all the elements much easier to break.