Wait, though, there was a transition period before that happened. Early on, it was serious art photographers who were the Pictorialists. That transitioned to the f64 school of thought.
The Straight Photography movement and thats because a cheap imitation of painting would never have brought photography to the point of being considered art and many photographers like Stieglitz and Steichen realized this and changed their pictorial ways and joined the likes of Sheeler and Strand and then out west Weston, Adams, Cunningham (f/64) in what finally brought photography to a serious art form by using the things that make photography unique from all other art forms.
The painter turned photographer Charles Sheeler.
" I have come to value photography more and more for those things which it alone can accomplish, rather than to discredit it for things that can only be achieved through another medium."-Charles Sheeler
For photography to move ahead at this time 1905-1920s it had to find its own voice and move away from trying to be something it wasn't. Outerbridge, Steiner and Evans all also realized this to in the early 1920s. Straight Photography was now alive and kicking and could stand on its own instead of trying to imitate and be something it wasn't.
"The arts equally have distinct departments, and unless photography has its own possibilities of expression, separate from those of the other arts, it is merely a process, not an art; but granted that it is an art, reliance should be placed unreservedly upon those possibilities, that they may be made to yield the fullest results." - Alfred Stieglitz
"Honesty no less than intensity of vision is the prerequisite of a living expression. This means a real respect for the thing in front of... the photographer... this is accomplished without tricks of process or manipulation through the use of straight photographic methods..." - Paul Strand
"Look at the things around you, the immediate world around you. If you are alive, it will mean something to you, and if you care enough about photography, and if you know how to use it, you will want to photograph that meaningness. If you let other people's vision get between the world and your own, you will achieve that extremely common and worthless thing, a pictorial photograph." - Paul Strand
Also wanter to add that many art historians also credit photography as being partially responsible for moving painting away from trying to do exact representations of things. The first photo was 1827 and by the mid part of the 1800s-1850s/1860s photography was becoming very popular and painters could now paint the way he/she felt and what their impressions of something was. They were now free to paint more the way the scene or the object felt to them because there was a machine (the camera) that could do it far better than any painter. Impressionism became a major movement in the 1870s so many think that photography had played a role in this movement.