The human eye can only focus on a certain area
Bad news - we have two eyes
A photo camera has one single lens - it can only see in 2D. No stereoscopic information can be captured with a standard photo.
A human observer on the other hand is looking at the 3d street art with his two eyes, thus seeing the street and the environment in a stereoscopic manner.
With is single lens, the camera will "flatten out" whatever it captures and puts everything on a two-dimensional plane, which is then seen on a two-dimensional photo print or - a two-dimensional computer screen. Everything seems to appear right.
Our eyes in contrary can only and will always focus on a certain limited area of whatever is front of us and give us a three-dimensional impression of our surroundings unless, we close one eye.
As a result, people standing on the same ground level on which a large 3d chalk drawing is painted will not see the dramatic 3D effect of common street art, the way it´s presented through a photograph with a special lens, at all.