Your question is a logical one that if analyzed is in truth pretty simple to answer. Peter Keating is a leech. He is a parasite that needs others to live from. He is neither talented, smart, or independent. He becomes a "rising star" in the architectural world because of Howard's Roark's designs and Ellsworth Toohey's promotion.
As with most people like Keating, the desire for fame and fortune pushes him to do anything it takes to acheive that end. He was a snake and he was promoted by Toohey as a prince among men. That is why he was thought to be selfless.
"Keating achieves success rather quickly by learning, under Guy Francon's tutelage, how to manipulate others. Harboring no illusions about his lack of creativity, Keating easily accepts the help of others; in some cases he actually puts his name on others' work."