Well, it is actually extremely expensive to produce customer-facing documentation of high quality. Vendors don't do that (or do to a limited extent), because a) this is big cost and time consumption, and b) because the makers don't request it anymore; even given such information of high quality they would not use it! Many of them have acquired the reflex of calling vendor's support for even the smallest question (let alone anything involved -- they will shift that job to vendor entirely), so the vendors have performed the selection of customers they can work with that way, and neglected the others (and the documentation).

From personal experience, the expectation of good documentation today for many developers -- an instant reply to a question limited to 140 characters.

I suspect it prevents potential customers from realising that their competitors' chips are better and have actual documentation until it is too late.

Not a really satisfying answer but I've been told it is to prevent competitors from reverse engineering.

Some fraction of the potential customers will call you, which gives you contact information. If you're proactive enough and good enough at selling to that fraction, the end result is beneficial for you.

I'd love to know whether that works...