> For the vast majority of jobs being absent a full work week without approval means you don't have a job the next week.
College isn't a job for the student, it's a training and certification program that the student is paying for. The accreditation agencies that validate that the program is appropriate for the certifications being offered generally do not require attendance tracking, because regularity of attendance isn't what is being certified.
> Why does being an adult university student mean you can be absent for months and still succeed?
Attendance tracking in schools for children serves a number of purposes related to the fact that the students are children, including that students are not entrusted to review the syllabus and evaluate the level of engagement required for success on their own. Most students can't be absent for months and succeed, and if they try they will fail. Attendance tracking isn't needed for that if your assessment, grading, and academic standards are working properly (and attendance tracking doesn't fix the problem if they are not.)
> And why do we need instructors?
Because those are among the educational resources students are paying for (though there are universities where that is less the case than traditional universities, such as the competency-based WGU, which has won high praise from many corners.)