Test banks are provided by text book editors to instructors using their text books. It has nothing to do with being lazy, it is a resource being provided to overworked faculty. Test bank questions from a good company have been researched and tested for usability, and are written with NCLEX style critical thinking in mind. There are some designated as critical thinking, concrete data, inference, etc. The instructor can pick and choose among them, selecting questions that reflect what she considers important.
Good test questions are hard to write, what seems clear to the author may not be so clear the test taker. In my master's ed classes, we had entire chapters on just writing test questions. And, the test has to be given several times to weed out or revise questions that turn out to be misleading. Writing hundreds of them for each class, especially when you have more than one text, takes many hours; and salaried faculty aren't really compensated well for all the hours spent outside of the classroom, preparing lectures, grading papers, advising students, attending mandatory meetings. I'm only adjunct, yet I can spend anywhere from 10 to 30 hours just grading papers for one of the classes I teach (a 2.4 credit course) every week.
OP: if you try to access a test bank for your textbook, without being an instructor; yes that is cheating. Practice banks and mock tests set up by the university are not the same thing; that resource is knowingly being provided to you. However, most test banks have far more questions than actually get used, by the time you've memorized all of them, you may have well have just studied the chapter and learned the information to begin with.