And I believe that reason is, not because of the usage, but in the early days, there weren't as many shards, so putting them into fg decks would only have increased the number of shards drastically, and considering they are more rare than weapons, as a developer, you wouldn't want your game rampant with shards in the early goings. But now, I don't see any reason why some of the fgs can't have their decks changed a bit to incorporate some shards. The community now has lots, and are fairly easy to come by.
And again, refer to the extra large text in my previous post. If you haven't played any FGS yet, then you don't know how much money you make for an average game. It is 4-5x more when you win against platinum and fgs you can win upped cards.
----TIP----
If you can't seem to win against platinum level, try a lower one. If you cannot beat bronze, then maybe your deck sucks.