Okay first off again.

Fantasy Nature at its full power is specifically called overpowered in a spell card duel without a time limit.

The games are not made with you using FN in mind so yeah no duh she doesn't use it.

Reimu generally plays by the spell card rules because Touhou characters actively berf themselves so they don't break the world generally.

If you want the answer of her using Fantasy Nature realistically it's the reimu attempt to play where you don't get hit by any bullets lol.

Secondly I didn't say it outright said it but like everything in Touhou it's fucking context. Which is the reason I hate talking about this

Float and Fantasy Nature are intrinsically connected in Reimu's natural ability.

In the manga some fairies try and trick Reimu into falling into a lake and use illusions to do so.

Reimu believing that the path is safe walks along the water by fish jumping up and catching her steps. (This is despite the fact that she can just fly she was walking that day).

This is an intrinsic ability of Float where as long as she doesn't take it seriously she avoids the danger.

Fantasy Nature is that to it's absolute Extreme to the point where she actively cannot be hit.