Flexible cabling for wiring
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.