Why can't I just give her a mine I found in the subway system and be done with it? Like, a mine is a mine bro...
The quest Wasteland Survival Guide is not about fetching the mine, but writing a book about how to survive in the wastelands. One thing you need to know to survive is to traverse a minefield.
The mine is not the objective, surviving the minefield (with proof) so she can write it down for her book is.
Because Moira is writing the Wasteland Survival Guide, and wants you to try and survive an existing minefield.
She also points out the following in advance:
I need someone to research how to travel through a minefield.
I've heard stories about a ghost town that's just full of mines. Traders just call the place "Minefield."
Sounds like the place for some fieldwork! Get in there, get back, and tell me all about it. And could you bring back a mine for my studies?
Since others have already addressed the question in the title, I'll address the question in the body:
why can't I just give her a mine I found in the subway system and be done with it?
According to the Fallout fandom, you actually can:
As frag mines are quite prevalent in the game, it is possible to lie to Moira about visiting Minefield and still complete the optional objective to complete the quest.
Do keep in mind that the outcome of this quest can change based on whether you lied to Moira or not.