Because the peaks and valleys of wave motion do not move. Only the vibrating medium moves, at the frequency of the wave.
Standing waves are caused by a pair of matched (amplitude/frequency) travelling waves moving in opposite directions.
I will try to find a website with an illustration or video of what is meant
Why standing waves are called standing?
2 answers
http://www.youtube.com/watch?v=3BN5-JSsu_4