It's hard to make it illegal, because under the current system someone needs to draw the lines somewhere, and as long as they can reason why those lines are in

It's hard to make it illegal, because under the current system someone needs to draw the lines somewhere, and as long as they can reason why those lines are in those places, it's hard to prove malice. Attempts to negate gerrymandering typically focus on making sure that those deciding the lines are as unbiased as possible, usually with cross-party committees drawing the lines instead of elected officials. The best option though is to remove the need to draw the lines at all.