Former college TA for physics here, I can tell you that not every teacher/professor does this by choice. My professor was obligated to use the textbook which included an access key for a homework site. He had to use this homework site (which already had the questions on it, he didn't make them), which meant students had to buy the hardcopy textbooks to gain access to the homework portal to do and submit the homework. He knew it was bullshit, I knew it was bullshit, the students all knew it was bullshit, but his hands were tied. He had no choice, and I could see the pain in his eyes whenever anyone asked why he assigned homework using the online service rather than good old fashioned paper and pencil.

Well, who forces him to use this system? Follow the money, you'll probably figure it out. This was at a very large university in the US, but I can assure you this happens at all colleges, big and small.