My company (and others in the same sector) depends on certain proprietary enterprise software that has literally no publicly available API documentation online, anywhere.

There is barely anything that qualifies as documentation that they are willing to provide under NDA for lock-in reasons/laziness (ERPish sort of thing narrowly designed for the specific sector, and more or less in a duopoly).

The difficulty in developing solutions is 95% understanding business processes/requirements. I suspect this kind of thing becomes more common the further you get from a "software company” into specific industry niches.