Because of this
Shinra Power Company
>ALL Files
>> SOLDIER FILES
>>> Combat
>>>>Materia
I assumed you ripped it from one of the spinoffs I hadn't played.

My point is very simple HC, even trying to explain it, you gave someone that in its core, is still magic. Magic replaced with the word materia. To make an explanation truly be an explanation you'd need to be highly detailed, even then the explanation wouldn't work in real life, but that's ok. That's what makes it fantasy. But vague explanations are nothing more than saying the process in which the magic is transported. When I have more time, I'd write up something that if included would've been a hell of a lot closer to an "explanation" then Square gave us.

Overall though, Square took the word magic and replaced it with materia, because it sounded cooler.