This is one that we debated quite a bit when we started to develop the algorithm, and the decision not to include abs as one of the main muscle groups came down to two things:
When most people think about abs, they tend to think about seeing their abs, which really comes down to getting to a certain body fat percentage (largely driven by diet). Given this, we felt that Strength Score wouldn't be the best metric for what people actually care about here.
Abs are trained a bit differently. While progressive overload can certainly apply to ab development too, and it can make sense to include some weighted ab movements that you gradually increase weight with, there is a limit to how developed most people want their abs to be - and from an absolute strength perspective this doesn't follow the same proportions as other muscle groups.