> like every single political party in power everywhere where gerrymandering is legal
Leaving aside the rather important specifics here, this isn't even true in general. Taking US, while there are examples of gerrymandering from both parties, one of them does it a lot more than the other.
Now, it could be cynically said that the difference is because one party has a much more substantial advantage to derive from it, because it is on the losing end of popular support due to demographic changes and urbanization, and it has to resort to it to maintain control of many state legislatures. Nevertheless, as things stand, it is a de facto partisan issue.