People want to have their cake and eat it too. Think about how raiding works in this game.

You pick when you want to raid.
No world contested spawns.
You don't lose experience.
You don't really lose gil (food stays with you, repairs are minor).
Gear drops every time, a set number.
You get tokens to pick what you want if your luck is bad.
You only need 7 other players.
You don't need to farm other enemies to "spawn" the raid boss.

As opposed to:
Raid times set by when enemies spawn.
Having to rush there along with hundreds of other players.
Losing experience when you die.
Sometimes raid mobs don't drop the "full drop" just maybe 1 piece.
No tokens for "participating"
Needing a dozen or more players
Spending days/weeks farming lesser bosses to spawn the "raid boss."
Dying to the raid boss just to wind up not getting any drops at all since you only get one shot.

The list goes on and on. The point is, while the raid content may be different, the environment is designed for casuals. Given that, just do it weekly like nearly every other bit of content in this game.

If anything, if raids could be repeated weekly for unlimited drops, it would go against the precedent the game has followed for 10 years.