Anyone who is good at SEO, good enough to help clients in highly competitive industries rank better, can theoretically build their own sites and work for themselves. Nevertheless, many SEO experts don't bother creating their own sites, preferring a steady income from working for an agency.
The most common reason SEO professionals cite is that they prefer to spend time with their family when they are not working. Working on a website that makes money takes a lot of time. Others say they tire of link building and creating content and want to do something different when they get off work.
Some SEO professionals have a site or two of their own, but agencies offer steady pay and benefits. Once a person gets a mortgage and has their family covered by their health insurance, it's hard to give a full-time job to work for themselves. In today's uncertain economic times, it feels foolish to risk everything for a possible income. Things change quickly in the SEO industry. A disgruntled competitor can create bad links to your site or a Google algorithm change can disrupt a person's income, even with the best SEO.
A few SEO professionals have their own websites, but they are rare. Those that do usually succeed, but they often sell the sites to make extra money because it becomes too time-consuming to keep them up. They often command high prices as they are affiliates sites with proven traffic and commissions.
Just because someone works in SEO and does not have their own websites, it does not make them a fraud or incompetent. When you interview a person to work on your site, whether they have their own websites is irrelevant. They are not going to work at SEO all day and then do it at night for fun. For more information click here https://www.reddit.com/r/SEO/comments/fc7e4r/seoprosofredditwhydontyoujustbuild_your/.