For many users, this seems like a deliberate anti-piracy measure. In reality, it is a security and data integrity feature built into the Ren’Py engine. This article explains why this happens, and more importantly, provides the definitive guide to creating a —essentially, how to bridge the gap between two different installations so your saves work everywhere.
If you want to sync your saves officially without manual file moving, look for the feature in the game's menu if the developer has enabled it: renpy this save was created on a different device link