I think there are multiple reasons for this, but they are mostly overlapping with preserving internal power structures.
PM's don't want anecdotal user evidence that their vision of the product is incomplete.
Engineering managers don't want user feedback to undermine perception of quality and derail "impactful" work that's already planned.
Customer relations (or the support team, user study, whatever team actually should listen to the user directly) doesn't want you doing their job better than they can (with your intimate engineering and product knowledge). And they don't want you to undermine the "themes" or "sentiment" that they present to leadership.
Legal doesn't want you admitting publicly that there could be any flaw in the product.
Edit: I should add that this happens even internally for internal products. You, as a customer, are not allowed to talk to an engineer on the internal product. You have to fill a bug report or a form and wait for their PMs to review and prioritize. It does keep you from disturbing their engineers, but this kind of process only exists on products that have a history of high incoming bug rate.