Editting simcity saves was my introduction to hex editing...

The trouble is that if some weirdness happens because of the edit, you've got to handle it even if you say it would be reasonable to assume that it's outside of being supported. Maybe you spend a bit more time defensive coding around what inputs it reads from the file, maybe a certain proportion of users doing the save edit see bugs in an apparently unrelated part of the game and seek support (and their bug report might not be complete with all the details), developers spend time to chase down what went wrong, maybe they bad-mouth it on forums which affects sales - there's going to be some cost to handling all of that.

One of the anecdotes from Titan Quest developed by Iron Lore is that their copy protection had multiple checks, crackers removed the early checks to get the game running but later 'tripwires' as you progress through the game remained and the game appeared to crash. So the game earned a reputation for being buggy for something no normal user would hit running the game as intended.