Why were wisdom teeth not eliminated by evolution?
Reductions in the size of our lower jaw that result in wisdom teeth sometimes becoming impacted result from changes in our diet, and are not the direct result of the evolution of a smaller jaw. This means that natural selection has had at most 8000 years to act on the phenomenon, which occurs in people in their mid to late 20s (who, historically, probably would have already reproduced if they were going to), and so the selective advantage of lacking wisdom teeth has not had enough time or been strong enough to cause the no-wisdom-teeth phenotype to spread throughout the human population. 

Global human mandibular variation reflects differences in agricultural and hunter-gatherer subsistence strategies - Cramon-Taubadel, PNAS (2011)

Before Agriculture, Human Jaws Were a Perfect Fit for Human Teeth - Smithsonian.com (2015)

Incongruity between Affinity Patterns Based on Mandibular and Lower Dental Dimensions following the Transition to Agriculture in the Near East, Anatolia and Europe - PLOS One (2015)