Why is a high-ranked organic listing more beneficial than an ad, since the Google ad
 

Why is a high-ranked organic listing more beneficial than an ad, since the Google ad will always be ranked up the top?

That's true, Google ad will always be ranked up in the top but the duration of that will last till you are paying for that ad as the duration ends the keyword will drop and the links collection may drop, so, to maintain the traffic and the visibility of the website one should follow the SEO.