Why aren't users enrolled in the courses when they're enrolled a curriculum?

When you enroll a user into a curriculum, the user is not enrolled in the contained course until the user launches the course in the Learner Interface. This has been done so that the user transcript and learner’s activity reports show a course’s accurate enrollment date, completion date, and time spent. It also allows admins to be able to send out the enrollment email for each course upon launching them for the first time.

When you enroll a user into a curriculum, you do not have to manually enroll the user into the individual courses, the user transcript will update as the user progresses through the curriculum.

If you would rather have learners enrolled in the courses when they're enrolled in the curriculum you can add post enrollments to the curriculum based on "When Enrolled" then add each course that's inside the curriculum.