Not sure, I assume it would be that easy, given the carts are literally just a chip inside.
Yes, but the roms are, since the DS, (literally) black boxes instead of the classic electronic definition of "rom chip"
The basic idea isn't different from SDs, USB drives, etc - you send some weird commands and you get back the data you asked for, wrapped in a weird format - but those are more or less officially publicly documented, while Nintendo game cards use their own undocumented protocol that by itself is already a challenge to understand, let alone copy - and that's before considering the encryption of data as it flows between card and console!
Nothing is impossible to copy - Nintendo somehow knows their inner design and builds them on large scale commercial level
- but as usual for game console mods, few people have simultaneously the technical background, the will to reverse engineer, the will to reimplement, the time for all of the above, a place with lax laws on the manufacturing of the flashcard, and the potential audience (As the red/blue button Sky3DS proved, there will [likely] be at least in part competition from hypothetical CFWs, and no guarantee newer games running on newer OSes will not have new checks)
(Also consider: WiiU, PS3, PS4, Vita took multiple years for the development of a way to install games from "unknown sources", yet nobody made a true flashcard/chip/ode that works without CFW and custom drivers in the first place; for Wii and 3DS this happened but most prefer installing them anyway)