Well maybe that is true for a baseball field but it is definitely not the case for a website.
Think of your website as a beautiful newsletter; you need to create it (which include design and printing) and you need to mail it. Building a website and then not doing the steps to get it to show up on Google searches for your products is equivalent to printing a newsletter and never mailing it out.
What is SEO
SEO (Search Engine Optimization) is just doing the needed actions to make your website show up on the internet for interested people to see. To use the newsletter example from earlier, SEO is the “mailing” service for your website; it gets your website out there in public hands.
Optimizing simply means to make as functional or effective as possible.