Also, good/real SEO usually requires content writers. Which is an ongoing process, and is probably not worth the cost to most small businesses.
With that said, most of the SEO companies that cold-call them are quite shady. Other clinics in the city have had their search rankings drop after hiring SEO services who use scummy tactics. The new "snake oil" perhaps?
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.
You touched on the other issue: Even if all you do is write and design, by calling yourself SEO, you're attracting clients narrowly focused on their search ranking. That's the metric they're going to judge your performance on, and it won't matter how accessible you make their site if they don't get the #1 spot for "cosmetic dentistry allentown pa." So by selling SEO, you miss an opportunity to educate clients on creating a memorable user experience.
