First, I think many here may be viewing Code Review as a Service from their frame of reference: from unicorn start ups, FAANG tech companies, prestigious universities, etc. Many of the companies that we help aren't coming from this world. They're small startups, looking for technical guidance. They're entrepreneurs who need to ensure they're not being duped by app developers. They're older, less tech-savvy companies that are looking to modernize. These companies need help, and they can get help from people who have technical expertise. (And by the way, some companies aren't sophisticated enough to set up linters or code scanners at their stage of development)
In a similar vein, many of the developers we help may not have had the same level of education, learning, or coaching as you or I may have. For example, I've helped introduce more modern syntax options to developers, such as string interpolation and extension methods in C#, to Options and Streams in Java, to filter/map/reduce functional patterns and optional chaining in Javascript. These developers may have never seen high quality code or had mentors who insisted on a high bar for code quality.
Even for more sophisticated customers, I've left comments ranging from security vulnerabilities (e.g. SQL injection), errors in boolean logic, recommendations for improved test coverage, recommendations for simplifying code (e.g. creating reusable functions), preventing race conditions, and more. I've also reviewed candidate assessments to help unburden senior engineers so they can focus on writing code.
Sometimes, the proof is in the pudding. There's a market for these services and that's why some companies pay for them and why I get to review code on demand. I periodically get feedback from the teams I help that I've done 'Nice Work', receiving positive ratings from the developers I review for. I'm proud that I can lend my expertise to make code better.