Correct, but that is exactly how the people talking about the self-publishing route are thinking: in terms of return on investment. The ROI in a standard publisher is small, and that is how it is supposed to be for something that is just a side project where you don't want to spend additional time. I am not saying that self-publishing doesn't have use cases: if you want to publish a book for which it is difficult to find a publisher, or if you're considering to do this as a full time job or a big part of your business, then self-publishing might be the right thing to do.
I'm sure it's how some people are thinking but there are other reasons you might want to self publish even if you take money 100% out of the equation.
- As someone else noted, you can put free eBooks online. If you're writing a book for visibility, being able to distribute free copies (or customized versions of free copies) might be a benefit.
- You don't need to conform to publishing industry length standards. If 75 pages cover the topic, you get most of the benefit of publishing "a regular book" for a lot less effort.
- You're on no one's deadline but your own. This has pros and cons but does let you better work a book around life, work, and other projects.
- The style, format, etc. is all up to you. Again, pros and cons.
I have made (a bit) more money with publishers than self-publishing. I'm also not sure if I'd go with a publisher again barring a strong reason to do so.
And you're going to be doing most of the marketing yourself in any case.
For most people, writing a book is an investment whether you go through a publisher or not. Unless it's just a passion project, you're presumably doing it for your career/brand/etc., not to make a meaningful amount of money. If you are evaluating how to go about a book project based on pure financial ROI, the short answer is you should probably do something else.