Because the Photography Plan is a deeply discounted bundle aimed at digital photographers.
Creative Cloud Photography Plan -- approx $10/month for 12 months. Includes the very latest versions of these great products & services: -- Photoshop CC and Photoshop on iPad -- Lightroom on desktop, mobile and the web -- Lightroom Classic -- Portfolio website + hosting -- Spark with premium features -- Bridge -- 20 GB cloud storage (upgradable to 1 TB). https://www.adobe.com/creativecloud/photography.html
If you only needed Photoshop for a 1-2 month long project, you would need to buy the costlier monthly single app plan. Different options for different types of users.
Nancy O'Shea, Adobe Product User & Community Professional Alt-Web Design & Publishing ~ Web : Print : Graphics : Media
The special photography plan is also somewhat like a 'loss leader' advertisement to get you into a grocery store to buy the items on sale, while hoping that you will decide to add other purchases... the photoshop in the special plan is exactly the same as the photoshop sold as a single product for a higher price
Another way to see it: Adobe offer standard plans for all single products $20/mo for an annual commitment or $30/mo for cancel without penalty. This is easy for buyers to understand, especially in business, though most businesses will want team or enterprise plans - and there is no Photography plan for teams. The Photography plan is for the price-sensitive consumer, and by pitching it low Adobe bring in millions of customers...