So many leave for greener pastures. You can do everything right in creating content, getting great links, having solid on page optimization, but no movement for months at a time. When you're solely SEO, you probably report to someone who starts to get antsy after a few months of "not ranking 1st for X term", so you end up switching to paid or something else entirely.

I've had most success in hiring copywriters who are eager to learn and then teaching them the fundamentals related to creating content-- keyword research, interlinking and thinking of who/how their content will either further business goals or help with generating backlinks. The technical component is easier- just look for anyone with Screamingfrog on their resume or be willing to teach someone who is semi-technical.