Adobe Express is a whole other animal. It's consumer level, browser-based web services are aimed at people with little graphic design experience who are mainly using it for social media. Likewise, the equipment requirements are minimal.
Lightroom & Photoshop are pro-level tools aimed at photographers and experienced graphic design professionals with the necessary equipment to support the software.
Different tools for different tasks & types of users. 🙂
Nancy O'Shea— Product User, Community Expert & Moderator
@default52xhz54ik24v , Illustrator is not in the Photography plan either. The only plan that includes all three (or any combination of two) is the Creative Cloud All Apps plan.
As to your "why" question, we are volunteers who are users just like you are, and we don't work for Adobe. If I were to guess, I would say it's just marketing, but that's my own opinion and not official word from Adobe.