Well, the simple correct answer is probably this: answers vary. There are multiple instructors, and with them there may be multiple reasons.
When I was a college instructor, I was told that the college had made certain promises about what the college would do, which included having students physically on the premises for a certain number of hours. Students who did not show up were hurting the percentages that the college needed to keep high in order to fulfill its obligations.
Edit: adding this paragraph, since I suspect this might not be easily implied by everyone. Checking attendance and following up with non-attenders, even of a single class session, may be an expected requirement of the job. For me, they were. Class sessions were 4+ hours long each, and instructors were even expected to try calling students' cell phones during the first class break, if they didn't attend. Different instructors, like many types of employees, had different levels of how much they may have fulfilled an individual requirement. Still, this simply demonstrates that "pressure from above"/administration may be one reason that may influence some instructors. Attendance was not directly graded (due to some government-related regulation), but there were requirements about how attendance would negatively impact grades under the category of class participation, and attendance could also affect the final grade by impacting additional grade percentages such as in-class quizzes. Some details may have been simply encouraged at some times, while being requirements at other times, and the simple way to be compliant was to just be strict (resulting in this being a significant requirement on students). Even in an institution that doesn't have as strictly enforced policies, there may still be requirements, or suggestions (which might be received as requirements), that might have influence on some instructors. (These influences may come from a certain college president, or department head.) Such influence might impact many years down the road, even after the instructor is no longer under the same supervisor.
Another reason is that students who don't show up are, well, not attending. This means that they are basically one step away from dropping out. (The only step away from being a drop out is not what they do, but how often they are doing it.)
Another reason is that a key purpose of the college education system is to provide the students with the preparation that they will be needing. As I've read this site, I've learned more and more that the precise goals may vary a bit between different institutions, particularly those that describe themselves as "research institutions" vs. those that describe themselves as "career preparation"/"technical colleges". Maybe some have some more philosophical/altruistic goals rather than corporate culture. Regardless, an element that is likely to be quite common is to want people to be successful in the organizations that they join after college. Most organizations do not want people to be failing to meet attendance requirements. If students can be exposed to a certain level of expected discipline, such as an official demand of attendance, that may result in certain character building that may serve them well after graduation.
Some argue that final exams would have their grades harmed by students who aren't learning, and so we can just rely on the final exams as a useful, accurate way of measuring knowledge. The counter-argument is that a student who skips classes may be a good test taker, and may effectively manage to demonstrate knowledge, while not managing to successfully demonstrate the discipline of needing to meet requirements other than just knowledge. Some employers don't treat the diploma only as a demonstration of accumulated knowledge and understanding of related principles. Instead, they treat the diploma as a demonstration of students being able to meet whatever requirements were placed on the student, which may involve some life skills (like scheduling) and not just accumulating certain pieces of "head knowledge". The educational institutions (or individual faculty members within educational institutions) may be choosing to cooperate with such methods.
Again, I refer back to my initial paragraph. These are simply some reasons, and one or more of them might be part or all of the reasoning that gets used by some instructors, while other instructors may have their own different reasons. So consider these various arguments as just being a sample, and not as the single clear-cut absolutely-right universal answer that everyone, everywhere, is actually using.