In which case this is your answer; they're worried about knockoffs, because without the firmware logic their devices are simple commodities, ie don't really have strong differentiators from the competition.

The other hugely important side of this is that a lot of these devices have the ability to transmit on frequencies which they may not be licensed for, or may not be licensed for in all markets.

The easiest example being b/g channel 13. You're permitted to use it for WiFi in most of the world, but not North America. Keeping the firmware proprietary and "secure" is likely an important part of their FCC/IC certifications.

It is required(1). The FCC requires locking down the ability to transmit into regulated frequencies.

1. https://apps.fcc.gov/eas/comments/GetPublishedDocument.html?...

Is their firmware really noticeably better or have more features than competing chipsets?

Yes. 802.11ac, for example, takes both a faster transceiver and a bunch of firmware support, and time to market with this features was a big driver of sales. And that was an ongoing process - ac is actually a set of features, and it's taking years for all of them to be implemented.

Similarly, cheaper chips often don't support optional performance-enhancing features at layers 2 and 3 (link and MAC) that boost performance without any hardware investment.