Space aliens may have expanded the genetic wealth of our planet by means of casual sexual encounters with the life of earth, but the accumulated evolutionary change has not happened overnight.  Their slow progress may be due to the limited numbers of such perverts.  The book of Enoch claims that there were 200 "angels" which came down from heaven and procreated with the daughters of men.[1] 

          With such a small population of perverts, the space aliens do not have the capability to effect large amounts of evolutionary change across all life forms at all times.  Instead, they must focus on a few species or genera at a time, and procreate with them for several dozens of generations, in order to effect a rapid evolutionary event.  The degree to which they can exert internal social control among themselves for the purpose of perpetrating such an event might be inadequate for continuous achievement of such endeavors.  Without the social discipline and cohesion necessary to accomplish a mutual goal of developing a certain species, each alien goes their own way, procreating with whatever species suits their fancy, the results of which are minimal because their single contributions to the gene pools of various species are watered down and lost in the greater population.  Hence, rapid evolution is infrequent because the group of space aliens effecting evolutionary change is too small and too disorganized to consistently accomplish premeditated speciation events.