The first time someone asked me that, I got caught in a mix of anger and confusion. I didn't have the correct answer right away, and I had to say overly generic things like "you also need luck" or "It's more complicated than that."
It’s the SEO-variant of “If you are so smart, why are you not rich”?
For that person, who maybe was trolling me, or perhaps she was genuinely curious about it, I have a more detailed answer.
The short answer is
To get rich, you need to create a business. And creating and managing a business requires entirely different skills than the ones you need to know for doing SEO.
It doesn't matter how good you are at SEO (or any other thing, for that matter). You won't become wealthy unless you are fortunate or learn how to build a successful business.
The long answer is
In my twenties, I created not one but two SEO agencies. I failed, not because I was a bad SEO, but because I lacked the skills and experience needed to build and grow a good agency. I had almost no capital, and I never had any formal training in hiring people. I never had training in sales or pricing. I never had training in accounting. I was young, naive, and overconfident. I tried and failed, even having strong SEO skills and a lot of satisfied customers.
At the end of the day, SEO is just an acquisition channel, not a magic wand to success.
Would I do something different if I knew what I know now? Yes. I would be more patient. I would spend a few years working in a corporation (like I do know) to learn things that I wasn't even aware of, saving more money for not having to eat the newborn company's earnings. I would market myself more aggressively (I was too embarrassed to do it, and somehow I still am).
I am not ashamed of my past failures. I know now that all those experiences make me now a more complete professional. I know a lot more about creating and managing a business -even after failing- than most people around me. That helps me think and imagine SEO with a perspective and completeness that not many can even comprehend.
The longest answer is
Many new SEOs start creating a website, often a niche site, and try to rank and get some money through affiliate programs or ads. Maybe with the help of a course they just bought, or going through on their own. They start learning to write content, upload it to WordPress and use a plugin to optimize metadata. They wander in places like BlackHatWorld trying to figure out the best links to buy or even decide to create some links for their own.
At this level, SEO is pretty simple and straightforward. Maybe you get a buck or two, and that is the moment when everything starts to get complicated. You don’t understand well why some articles work while others don’t, so maybe you have to learn the fundamentals of web analytics. You get obsessed about rankings, and run a Rank Tracker twice a week to see any progress you could make.
For most people, I would say that this is the point where you can go on your own, with your own SEO knowledge. To grow to the next level, SEO knowledge is less important as the ability to scale. With simple math you know that if you are making U$10/day with 20 articles you wrote by yourself, you aim to get U$100/day. But writing 180 new articles will take too much time, so you reinvest your little earnings in buying content from some marketplace. You need to learn how to outsource content. Strictly speaking, that is not an SEO skill.
Your website has grown a little bit. The shared hosting you have isn’t enough. You have to decide between moving to a bigger -more expensive- hosting or trying to optimize the wordpress website. Scaling a website is important in SEO, but not an SEO skill by itself. You have to decide if you will create more small websites and rank them, or focus on just one site. What? Business decisions? Another not-SEO skill that is needed to grow and become rich.
I think I made my point. At the moment you grow, having more people, capital and a diversified set of skills become more and more important, and SEO knowledge is less relevant. Keyword research or analyzing content or implementing technical SEO is secondary. Negotiating affiliate partnerships, hiring or outsourcing people, scale websites or learning how to sell is way more important.
In this post I talked about my personal experience in two different scenarios: creating an SEO agency, and going as Freelancer/Self Employed SEO. The third path is working for a company. That is a case that we do not need to discuss, because you will never get rich working for others.