Investing in SEO is a long term investment to promote your online presence. PPC advertising is just a short-term solution, not to mention too expensive to engage in for the long term. SEO, in comparison, is a better marketing plan that pays off handsomely in the long run.
Your campaign stops when you stop paying for your PPC advertisements. This is not the case with SEO, as the effect generated from your SEO campaign lasts even when you have stopped paying for the campaign.
Moreover, SEO is a more trusted approach to online marketing. According to a 2012 study, 86% of web searches prefer SEO listings over paid or sponsored PPC advertisements.