Article 400.12 forbids the usage of flexible cords for fixed wiring, but what makes an NM-B cable made out of stranded 14ga THHN non-flexible?
Terms 'cord' and 'cable' are used interchangeably in that particular article.
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Cordage is a very specific thing. NM is not that thing. It's that simple.
Again it's back to 110.3 and the requirement to use items consistent with their labeling and instructions. And frankly, you would do well to simply follow standard best practices.
Why are you asking all these "why" questions? They are off-topic here, we are here to answer bona-fide home improvement questions from people actually fixing their homes, not ouiji-board NFPA's intent with this part of NEC or another. Try on electriciantalk or mikeholt.com etc.
Like I say, we get this from EE types all the time. Part of it is classism: many think because an EE is a college degree, and an electrician is a voc-ed apprenticeship, like EEs are better or smarter somehow. Another misconception is that Code electrical is a subset of electrical engineering, not at a practical level, no. Code electrical is 90% packaging... Aiming for safety, serviceability, clarity and replicability. Another misconception is that Code electrical is simple, no, even a little research will reveal a bunch of stuff you don't know.
You also have swiss cheese syndrome, because you're trying to read NEC without understanding how it all connects together, and using the Web to fill in your knowledge but it's only giving you what you know to ask for, hence your knowledge is swiss cheese - full of holes.
Keep in mind NEC is the single book that defines how nuclear power plants, cyclotrons, auto assembly plants, hydro stations, datacenters and malls are wired, aside from homes. That's a big part of why it's so darn confusing and it's easy to get lost in irrelevance. It's also easy to get lost in the details and forget the basics, like 110.3, everything needs to be listed for its purpose.
Seriously, put the darn NEC down, it's too complicated for your level. Hit Home Depot or the library and get some practical consumer-aimed books on home electrical. Spend a lot of time reading on the above forums, Ecmweb, etc.
This stuff takes time.
The cables are made differently, specifically the insulation is different. Flexible cords have thermoset, or thermoplastic outer insulation. Where as NM cable has a polyvinyl chloride (PVC) jacket.
I believe the stranding is also a bit different, making the conductors in flexible cords more flexible (but I could be wrong about this).
There are several reasons flexible cords and stranded nm wire are not the same. Several comments on the code are accurate but the base reasons are not there. Wire count and how made is the largest single reason the type of annealing and count of the strands you will notice with cords the strand count is much higher and the wire annealing is slightly different to provide a softer copper. The obvious point next is the insulation pvc used on nm wire will crack from fatigue quickly when flexed where thermoplastic can handle hundreds of bends (I just chose the insulation that looks similar to nm) . From a code stand point I have seen inspectors allow SOOW & W. Flexible cords for temporary connections while permanent facilities were being repaired. Temporary repair is much more expensive but keeps manufacturing running because when done the cords are removed. I'm my area 90 days is the max.
NM from 14 to 10 gauge, as a rule, is made from solid wire (even though it's not required to be -- the NEC allows stranded NM in all gauges, see 334.2), while cordage uses stranded wire. Also, the jacketing and insulation used in cordage (usually either a thermoplastic or thermoset elastomer) is more flexible than the PVC jacketing and insulation used in NM cable, and even with stranded wire in both, the cordage can use a softer, more flexible copper.
If you did find stranded NM in 14 through 10 gauge (which'd be odd), you could use it anywhere else NM could go, but it probably still would lack some of the flexibility of cordage.