STABILITY. “Because transposon movement can be destructive, it is not surprising that most of the transposon sequences in the human genome are silent, thus allowing this genome to remain relatively stable, despite the prevalence of TEs. In fact, investigators think that of the 17% of the human genome that is encoded by L1-related sequences, only about 100 active L1 elements remain.”
http://www.nature.com/scitable/topicpage/transposons-the-jumping-genes-518