Why don't they just sell gil in the mog station?

[Fenwick]

If you allow people to buy gil, people will buy gil. It will inflate the market, and individuals who don't buy currency will eventually be unable to afford anything without a longer and more tedious grind that will continue to grow for as long as people inject more and more gil into circulation.

Edit: As far as Cash Shops go, personally I'd prefer vanity items to be implemented as game content. There is no need to throw money at something pointless, integrate it into the crafting/FATE system and give your players more to strive for.