University computer accounts are often surprisingly complicated
In many organizations, the life cycle of computer accounts is theoretically relatively straightforward. Your employees have an official HR start date, at which point their account comes into being, and eventually they will have a departure date, at which point their account goes away. There are wrinkles, but you can mostly get away with driving your account system from HR data. Often it's explicitly a good idea to do this, to make sure that people who are no longer employed also no longer have computer accounts (or at least access to them).
This is not how it goes in a university, at least on the research side of a university department (the teaching side is more straightforward). To start with, people have all sorts of relationships with the department that creates computer accounts; there are professors, graduate students, staff, postdocs, research assistants and research staff hired by professors, undergraduates doing research work with a professor or a group, research collaborators from inside or outside the university, visiting researchers, and more. Many of these relationships are not recorded in the university-wide HR and student information systems, and the university doesn't necessarily want them to be (especially for things like who is collaborating with who for research). Some of these people don't have any formal association with the university that would be reflected in university-wide HR systems, as they aren't being paid by the university, attending as a student, or otherwise officially recorded as being eg a 'visiting professor (status only)'.
When people have relationships that are officially recorded, such as being professors or graduate students, the start date of their official relationship may be well after we want to give them a computer account. For example, when new faculty are hired into the department, they have an official start date that may be some time in the future, but the department usually wants to start integrating them right away, so they can both feel welcomed and hit the ground running. The same is true for end dates in many cases. For example, just because a student has graduated doesn't mean that they've stopped interacting with people in the department and should be cut off by having their computer account closed. Some sorts of official relationships can go on hold for a while, such as a graduate student taking a leave of absence, and when this happens we definitely don't want to remove their computer account.
(In addition, the end dates of even the formal associations like graduate students graduating can be uncertain and changeable. People usually don't finish their thesis and thesis defense on an exact schedule that's known well in advance and never changes or slips.)
Complicating the life cycle is that people frequently move back and forth between different relationships with the department. In an extreme example, an undergraduate doing work with a professor can become a graduate student, then a postdoc, then a remote collaborator, and then come back to be faculty themselves. Graduate students can be hired as (part-time) staff, and then sometimes they become full time staff and no longer graduate students.
Visitors are their own collection of complications. Visiting researchers may be here for an extended period or just a flying visit where they're around for a week. They aren't necessarily from another university, since plenty of research is done in industry. They may have already collaborated with professors here and been given computer accounts, or they may not. Currently, they're often not entitled (by the university's standards) to have a university-wide identifier. Even if they are entitled to one, they may not be here long enough to go through the process for getting it (or be able to wait that long). And once they leave, we generally don't want to just delete their computer accounts, because they might either come back later or at least become collaborators.
A general theme of all of this is that the research side of an academic department runs on a broad network of relationships with all sorts of people that are developed and cultivated over time. Generally one of the last things the department wants to do is reduce those relationships through things like denying computer accounts or removing them. When graduate students or postdocs go off into the world, when visitors go home, and so on, the department wants them all to feel still connected to the department as much as is reasonably possible.