Perhaps they think that by forcing everyone to go through the sales dept just to get the basic docs, they'll get better sales opportunities.

How do you upsell a hardware engineer who just wants to buy a specific chip, and already has everything to evaluate and use it? You don't. So you force everyone to go through sales, and then sales wants to talk to non-engineering higher-ups, and then the upsell happens - while the people who actually knew what they wanted remain as far away as possible.

And if you don't have the pockets deep enough for the sales dept to acknowledge your existence, then you might as well not exist.

When you get into high volume production, the vendors have their own engineers who will work with you directly. Depending on the arrangement and your volume they might bill you for the time, require you to buy blocks of support hours, or they might bend over backward to help you out in any way possible if you’re buying enough parts from them.

Dealing with small clients is not a priority for most part vendors. Many of them won’t even sell you chips at all until you can qualify yourself as a big customer or, in some cases, buy a license to start designing with their parts for six figures or more.

Unfortunately for the small players, it’s not a priority for most companies to support small customers who might only buy a couple thousand parts or less.

Because creating good documentation suitable for external consumers costs money, and even if you have decent documentation, people buying small quantities of parts can end up costing companies money just to interact with one of their application engineers. If you're not set up for it (very few companies are), it literally is negative value to answer support questions for small time customers.

Because the margin per chip is low and supporting more customers cost more. They sell these parts for couple of bucks/part at volume, although it costs 20-30 million NRE. With the fixed production costs added, they have tens of cents per part margin. At that point less than 1M part customer becomes irrelevant.

Guessing: if you publish too much about your design, competitors can use it against you during sales negotiations. "Don't buy theirs, ours has better pin placement, better IF, better thermals, etc"

I think the short answer is because manufacturers don’t care.

There's certainly cases where the documentation basically doesn't exist, or it's essentially the same as the design documentation, and the strategy is basically that if you're a big enough customer then you'll be getting some engineer time to make things work, and if you're a smaller customer you'll be told to pound sand anyway, it's not worth them fixing up their documentation to get your business. Generally the more complex and specialised the hardware the more it'll look like this, and it basically saves them on R&D because they have only a few applications to actually focus on.