Why SEO is ongoing work
The engineers at Google update the rankings on a daily basis according to the set of algorithms they follow. Even if your website ranks #1 for few months during the SEO process, when you stop working on the site’s SEO your ranking will fall. This proves that Google likes websites that are updated regularly and if your competitor works regularly on their SEO when you stop, Google is going to rank them higher.
Therefore, SEO requires patience and consistent work to bring your website at the top spot and keep it there always. Though SEO is an investment, its results will last you long term.