For the same reason that people choose to get a job instead of starting their own business.
I agree with warrior's answer and would just want to expand on it to help it sink in if nothing else...
The conclusion that draws out this curious question is very valid so it might help to imagine a similar scenario and how it might apply, for instance: If I love to cook, I might want to open a restaurant. I can either go to culinary school or jump right into trying to open one (if money wasn't an issue AKA the '
cost of entry' was similar to starting one's own website) whereas another option, aside from culinary school; would be to get a job as a line cook to build my experience, to learn the business, to fill in the blanks of what I knew I didn't know yet or in some cases, because I would discover there was more to it that I didn't know. Then, when I feel confident and I know enough or
think I know enough, then I might go give it a try or I'll realize along the way how much more I've yet to learn or would feel I'd need to know about opening and running my own restaurant before being comfortable enough to be do it on my own. For instance, I might be exposed to managing beyond kitchen staff, the front of house staff (aka servers, hosts, bussers, etc.) where each one works differently, something I might never consider had I pursued the idea of opening a restaurant based solely on my passion to cook.
So to tie it more together, what you may be witnessing is simply a content writer explorer the world of content creation. In concept, it's simple to say that writing content for a website is arbittrarily easy to build website traffic, but some recognize the other steps necessary to build and maintain a successful website. Your question will naturally find answers for itself if you deicde to give either one of those curious paths a try...
Best wishes...and stay curious!