Why isn’t computer science considered an engineering field? What does it take for a discipline to become an engineering field?

Computer Science is a discipline of pure science that builds the foundations. Computer Science is too vast to be used in a single domain on its own. It’s similar to math and physics. Though math and physics are the basis of engineering, they are abstract and speculative pure science topics.
Robotics is a field of engineering that draws heavily on Computer Science in the fields of Artificial Intelligence, just as mechanical engineering does in the field of mechanics. Mechanical engineering is impossible without physics. Robotics cannot exist without computer science.
When a theoretical notion (AI) is implemented in the actual world (Robot), it transforms into an engineering technology (product), which we refer to as Robotics.

It will stay abstract till then. Engineering is a real-world application, whereas Computer Science is a notion.