Marketing employees can certainly have SEO skills. The biggest reason to hire an agency to handle all your SEO versus a marketing employee who’s in charge of all your company’s marketing is pretty simple. It’s cost. A seasoned marketing professional will require an average salary of $60K/year + you have to pay them benefits, vacation and sick leave, supply them with a computer, and countless other expenses. An SEO agency though, the average cost will be around $22K to 40K a year and you don’t have to pay them benefits, they don’t take vacation, they don’t require training in the industry and they guarantee their results.