It's not as though there haven't been issues with gaming these tests to a degree for a long time. Almost certainly this has been an issue of walking backwards into a big mistake little by little, which is the typical process of making big mistakes. You start by figuring out how to game the system right at "the line" as much as you can, then the day comes where you decide to be more explicit about it and build it into the software. You probably decide that you just want to play around at the edges, with incremental differences between "cheat mode" and real driving in the same realm as the bit by bit gaming they've been doing already. But once the system exists the natural process of optimization would push it to the limit of what was capable, and it turned out that was a lot. But by then you're committed, and it would take a lot of fortitude to walk back from such a mistake.

I'm sure the same sorts of dynamics have played out in many other auto makers, though it's notable that some, with a more developed sense of ethics seemingly, have not gone down that road.