If all they do is write interesting content and promote it, why do we call it SEO? Last time I checked, that matches the job description of public relations writer.
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.
It goes a little beyond content writing. At a certain level of resolution, SEO (the good stuff) and accessibility become nearly indistinguishable as well. So yes, there is a sense in which "real" SEO is merely a matter of writing web pages that give your users what they want with optimal markup, and SEO is probably a bad name for it - except that the people who need it are usually going to be searching for ways of getting good search results rather than advice on how to build more informative, accessible web pages.
I worked as an SEO for years. Quit because of this. "Good" SEO is about good content, good coding, and good usability. But in order to do that, you need great designers and writers. SEO people tended to be neither. My colleagues got into the industry by keyword stuffing and link farming. They were trying, unsuccessfully, to change their specialty after updates to Google's algorithm. We helped sites get ranked, but most of our clients would see a better return on investment hiring designers and writers.
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.
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.