Really? I know professors who wrote textbooks in their spare time. Universities could give professors a paid year off (no teaching responsibilities) to work on textbooks. Time is a non-issue here.
"...or skills to create diagrams..."
Professors, who spend their time writing research papers that are usually filled with diagrams, don't have the skills to do so? That's an interesting assertion. There are plenty of software tools available for creating diagrams.
"handle formatting and typesetting issues"
Hm, I think there is a tool for that one too: LaTeX. What, it's too much to expect a professor to know how to use LaTeX? Well, there are several other tools that could be used. These problems were more or less solved decades ago. Professors routinely format and typeset their articles; formatting a book is not substantially different.
"write hundreds of homework problems"
I know professors who do this: they write homework and test problems for their courses.
Really, I think you underestimate what an experienced professor could do in a year. Even if what you were saying was true, it would not be hard to solve: universities could hire experts on formatting and publishing to work in their libraries and assist professors. It is also not necessarily the case that every professor would have to write whole books; if books were being shared by universities via their libraries, professors could make corrections and improvements to books as needed, with the changes being propagated in a manner similar to patches in open source projects. We have all the technology that is needed for this, we only need to put in the effort to organize such a system (but I won't hold my breath).