I suspect the reason is that outbound links drive traffic away from ads and internal links. The more visitors you have, the more profit you lose by adding outbound links... It's sad and I wish there was some way to fix the incentives.
That's exactly what people in charge fear in my industry (working in online publishing). The do fear, that the people find the linked ressources better, so that they may never even return.
Kind of like the fear of becoming obsolete. But instead of testing and maybe even becoming better, they just forbid us to link out.
