Some professors in US colleges are focused on research and publishing. Their pay and whether or not they get tenure is based somewhat on the amount of research they have published and the amount of grants they have gotten.

So teaching isn't a high priority for some professors as teaching is time that you could have spent doing research.

Some classes are taught - at least in part - by TAs or graduate students. TAs also grade most of the homework. In my school at least, I think the professors graded the tests.

Now that I think about it the TAs might benefit the most from these pre-made slides and such.