Why do scientists say that the environment selects the traits that allow an organism to survive and reproduce?

Short answer: it doesn't. Natural selection is an effect caused by the reproductive success or lack thereof of individual strains. Random variation will produce traits A and B. Organisms with trait A will be more reproductively successful than organisms with trait B. This results in trait A achieving dominance in the population, while trait B fades from the population. We then say that natural selection has occurred.