This forum is answered by volunteer Photoshop users. We are not Adobe staff so cannot answer why Adobe made that package, I'm sure the marketing guys would know why they did it and it has been very successful, so just take advantage of it 🙂
Adobe offers a single-app plan in addition to the all-apps plan for those who need just one app. The single-app plan can be annual (with a fee for early cancellation) or monthly (higher cost per month, with no cancellation fee). Photoshop was not excluded from this single-app plan. If someone needs Photoshop for one or two months, they might want the single-app plan.
When the Photography plan was first introduced, Adobe gave a deadline for getting into the program. Then they extended the deadline. Then they removed the deadline and just kept the plan.
As Dave said, we are users and don't know the "why". We only know the "what". 😊