The documentation they have may not be complete or in a form they are willing to release (e.g. because it mentions future products) They also may be reluctant

The documentation they have may not be complete or in a form they are willing to release (e.g. because it mentions future products) They also may be reluctant to release it because of fear of litigation. They may do something that’s patented, similar to a patent, or deemed patented by someone else, and releasing documentation makes it easier for others to find such cases. Also, in the end, there’s dollars. Even the simplest process to release documentation costs money (and theirs wouldn’t be the simplest for the reasons mentioned above). They may think there’s not enough money or reputation gain in it for them to do this (and for reputation gain, they would have to keep doing it)