Some of this is because the manufacturer considers the information to be proprietary and also because they not setup to support large numbers design teams. Some IC's especially early revisions are buggy and poorly documented so a lot of hand holding is needed[1].
[1] Think of a buggy piece of silicon where you need to implement a number of work around's in firmware. And where each customer invariably runs into an errata you didn't know about.
>Some of this is because the manufacturer considers the information to be proprietary and also because they not setup to support large numbers design teams
No, it is what is called a pain sales tactic. You make a person to go through big pain, spend lots of resources just to get a devkit, so they would not be inclined to throw it out midproject because it sucks
So you can usually just get the WiFi chip (for example, Broadcom or Marvell) on a SDCard form-factor extender, plug it into your standard SD Card slot and work on it with the SDIO controller on your development board.
Note: the SD interface controller must support SDIO and not just the memory SD-Card portion of the spec.
But, of course, you still need the development documentation and internal chip information. That, unfortunately, must still be paid for.
But apparently missing is information on how to program the internal registers, how to Tx and Rx data and wireless management information, which is probably what the Broadcom drivers require to manage the chips.
Basically, the internals of the chips still look like a 'black-box' to the drivers.
Of course, I may just be wrong.