Overall, SEO is a long term investment because it involves optimizing various areas of your website. Not only do you have to optimize your individual pages for keywords, but effective SEO requires building up your site authority, fixing technical issues preventing your site from ranking, improving the usability of your webiste, creating useful content, and more. Unlike PPC, which sees immediate results, SEO takes a couple of months before you start to see the ROI from your efforts. However, while PPC only lasts as long as you pay for ads, the benefits of SEO can last indefinitely.
This is why, despite the fact that SEO is a long term investment, many companies choose pursuing an SEO strategy over other marketing strategies.