I didn't graduate from a university, so this question is more on behalf of friends of mine who have had the experience of waiting for seemingly ridiculous(in my opinion) periods of time for an official transcript to be processed. It has been particularly frustrating recently for a friend of mine who didn't realize that a background check from a new employer would require her school to produce an official transcript. The school didn't return calls to the background check firm until my friend got in touch with the school through a different channel, only to be told that the transcript would be produced in two weeks and the process could not be expedited(no exceptions).
The frustration is beside the point though. Why must it take this long to produce an official transcript? From what I can tell, an official transcript is one signed by the registrar of the school and is not a photocopy. I could see it taking a few days for a graduate, but weeks? It seems like a common enough thing that I'm wondering what the explanation could be.
The simple answer is that an official transcript is a legal document and hence the bureaucratic procedures that are implemented to get them to students favor making sure they are above suspicion rather than expediency. That underlying rationale can be the cause of such a variety of specific reasons for it to take so long that I don't think any more specificity would apply universally.