You would be surprised. University of Michigan Publishing, one of the largest and wealthiest of the publics only has a staff of ~30, some of whom are shared with other departments. The university's annual contribution is well under $2 million. The entire University Library, the second largest research library in the U.S. after Harvard, has an annual budget of $64m (0.58% of the entire budget).

I think historically the larger presses were expected to be close to revenue neutral. They do well enough to limp along, but don't get the resources really needed to build new infra. As a cost center without significant tuition or grant funding, the library has little pull when provosts form their priorities.