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You can make 100 landing pages with interesting content on them, but if they are using words that people don't use when searching, they are less likely to be surfaced at the top of the results.

Those pages can be scattered across the site architect and have improper linking. They can have improper titles, meta descriptions, and image alt tags. Often times writers can't grasp these things, or they are so overworked with pumping out content that they simply don't have the time for them.

SEOs can be more technical, and can also make changes across a large site or networks of sites. EG: Missing the city and state from the h1 of every vehicle landing page for hundreds or thousands of vehicles on a thousand dealership sites can have a dramatic impact.

In this sense many SEOs also have the skills to be what we now call Content Strategists. People that do research, plan, and keep the content teams moving in the right direction.