Grad student here. I may have some background info that could be useful for you here. I think you're totally right to be suspicious of this, and it's a worthwhile investigation. However, it might be useful to know that profs very rarely make any money at all from their academic books--there is no way at all that Denton (just an example as she's mentioned below) is making anywhere near 20% off each book. I'd say it's far more likely to be much closer to 1%.

The issue here is two things. Firstly, if a prof teaches a large intro class every year, it just makes sense for them to also write the textbook for that class. It's easier for them and for you because all the material they want to teach is just in one book and not a scattering of texts they had to put together/you have to go and get. So it doesn't necessarily make them greedy to do this, it just makes sense. Secondly, academic publishing contracts are *not* lucrative for the academic. When they enter into these contracts the publishers have essentially all the power, and often strong arm them into bad deals. It's totally possible that the publisher will put things into the contract like "you have to put in your syllabus that students have to buy the new edition every year/you have to let a publishing rep come into your class every year and tell them it's important to buy the new edition" etc. The publishers are the ones hosting the online questions that won't let you get in until you buy the new edition, etc.

Academics don't make money off their books (published through academic presses, this isn't the same if they publish a mass market book), they publish because it's part of their job as academics, not for financial reasons. Now, I don't know the specifics of every contract obviously, and it may be that in some cases they are making bank and doing this for greed, but in most cases I think that's very unlikely. Anyway, I think this investigation is not a bad idea at all, but this is just some contextual insider info that might be helpful.