They must not get very much rain there (normally).
They’ve got a bit of a rainy season, but you’re right. And because of the terrain in most of Greece, the water doesn’t slow down much before it drains back into the sea. Arable land was a premium outside of the Peloponnese. Near Orchomenos, at a site with the ‘modern’ name of Gla, the Mycenaeans built a dyke to channel a river all the way around the edge of what was then a lake, drained the entire lake using tunnels (digging those must have been a nice fun job) and evaporation, and turned the whole redeemed area into farmland. During the turmoil at the end of the Mycenaean period, some invader breached the dyke. And this was centuries before Islam.