That question is about as meaningful as "why is not everyone a successful actor / investment banker?". Answer: not everyone is interested, qualified, and lucky enough to encounter the right circumstances at the right time.
I don't know this RadBrad, but I really doubt all he does is uploading casual gameplay to YouTube. I think being a successful YouTuber is very hard work: you have to stay active and interesting and on the forefront of developments, be nice to your stupid audience and give them what they want, and you need support staff for video cutting, artworks, giveaways and all this shit. Also it's neither a secure long-term occupation nor a life I'd like to live for more than 1-2 years (cf. Goose and other Twitch celebrities burning out). Do you know any streamer / gaming YouTuber that has maintained long-term success? Because I don't. And if you want to change your career afterwards, most recruiters won't give a shit about that YouTube and Twitch experience on your resume.