But you're not "spawning reality".Why? What you mean? To me it would be unimmersive to able to spawn reality around me at will, if that is not what the character can do.
Consider this: in a book that you are reading - a work of fiction - the author might refer to trees. And given that other features of the book make you think the fiction is set somewhere more-or-less temperate European, you probably imagine deciduous trees. And if the fiction also says or strongly implies its spring or summer (eg there's no reference to snow or cold winds, there is reference to sunshine and golden crops in the fields), then you will imagine leaves on those trees.
That is not you "spawning reality". That is you immersing yourself, imaginatively, in the fiction you are reading.
Now, suppose that the GM tells you there is a tavern. Are you expected - contrary to all intuition - to refrain from imagining anything? For my part, and I am guessing @TwoSix's also, I imagine stuff that flows naturally from what the GM has said, and other established or implied fictional context - so a fireplace, tables and benches, patrons drinking ale, etc.
And so when I then declare "I punch the nearest guy in the face" I'm not "spawning reality". I'm sharing my imagination, and acting on it.
To have to refrain from imagining anything until the GM tells me - eg there's a tavern, but it has no particular look, no particular people in it, no particular furnishing, etc - is extremely un-immersive. It's not how I engage with written fiction. It's not at all intuitive.