There are many reasons to do so, among which is, the algorithms that search engines use constantly change.
SEO professionals keep up to date with all these changes and can use sophisticated tools and analytics to ensure that you are getting measurable results.
There are many different variables that you must continuously monitor, track and test.
More importantly, a lot of what goes into SEO cannot just be simply automated and run on auto-pilot.
The time and effort you’d have to spend keeping track of Google algorithms and other variables that impact your search rankings could be spent focusing on your core business.