There are several reasons why a student would want the slides, and several reasons why a professor might not want to make them available.
For example, I can add a Dilbert cartoon to my slides, and, so long as I'm only showing them in class, that can fall under the umbrella of Fair Use. Make those same slides publicly available, though, and I may have just committed a copyright violation.
If we want to give the benefit of the doubt to the students, we can assume they find these slides to be a convenient way to review major topics. If we take a more skeptical view, however, perhaps the students are trying to "shortcut" the educational process.
I've been surprised sometimes at the number of students who tell me, "Sorry I could not attend class yesterday, but I'll make sure I look at the slides," as though looking at the slides is almost as good as the real deal. (A lot of my slides contain visual prompts that remind me of topics I want to discuss, and little more. For example, if I want to discuss the Denver Airport fiasco, I might post a picture of an airplane on a tarmac. Good luck, absentee, on figuring out what that sleek 737 represents or means.)
One of my students once remarked, "Your slides are really good in class, but not so good when studying for exams." I smiled, and informed the class that my slides were intended for me to use as a presentation tool, in order to help me lecture more effectively – not as a study aid. I'd be put off if students demanded my slides as though they had some sort of right to them.
That said, I do understand there can be legitimate reasons to use slides when preparing for exams. Occasionally, I have built two sets of slides for each lecture: one "juiced up" version to use while lecturing, and another "pared down" version to give to the students; that can be a nice compromise. (Of course, it can take a lot of time to develop two sets of slides, so I don't always do this. In this business, everything is a tradeoff.)