If EVERYTHING is in the slides, there are some major issues with the class. First off, including all the details in the slides means you either need long slides (that the professor will have to rush through), or each slide will be much busier, including more details than before. Either way, these are some of the worst types of presentations I've ever seen. Slides should give the basic idea, some of the details, but not every last analysis of the problem etc. Also, when teaching, effective professors will base how much time is spent on slides by the classes reaction etc. In addition, there are readings that are expected, and may not be fully covered in class. If that is too difficult for you to handle, you probably shouldn't be in college in the first place. The goal of college is for you to learn, and to learn for yourself. Not to illustrate that if spoonfed knowledge for 4 years that you can remember enough of it to pass the tests.
In addition, at least in engineering there are many example problems that would just be too difficult or awkward to do in powerpoint. Part of watching someone solve a problem, is watching the steps they go through to solve it. Sure this might be doable in powerpoint, but it would be a waste of a professor's time to spend a couple hours making the example in powerpoint when he could write it down on paper in 5 minutes. Plus many professors like to ask their students for input when solving example problems. At least in computer engineering there are often multiple solutions that are good, so having a powerpoint reduces you to a script that you can't violate. A good professor should be able to show examples that are targeted for the students, which often can only be done on the spot.
One last thing, part of why professors say "not everything will be covered in the slides" is because they want to make sure that after a test they don't have 10s of students coming up to them whining about how "this wasn't covered fully in the slides", or more likely "show me where we covered XYZ in the slides". Students can be whiny at times, and if you don't make these disclaimers, you'll be in trouble. Plus sometimes test questions are very similar to homework problems that weren't fully covered in the lectures. The ideas were likely covered, but not the exact type of problem due to time constraints.
Sorry, but courses cannot always, and should not cater to the students who don't want to go to lecture, skip homeworks etc. If you don't put an effort into it, it is not the professors fault if/when you struggle.
Phil