"Superstar industries" follow extreme power law distributions, where, due to the low barrier to entry, high ceilings, and limited consumer base (there are only so many hours people can spend on entertainment), the pie isn't growing and new entrants aren't going to be able to carve out much that hasn't already been claimed.
It's not a new thing, the name comes from the music industry which acts the same way. Anybody can record a song and print it on CDs, but the market is already saturated: you have to either really really really stand out like nobody has in decades (An event so rare I couldn't find any examples), or you have to get the support of one of the big players (game studios or music labels) to lend you their resources and audience.
Nearest example I can think of, mySpace was where she got noticed so that dates her breakthrough to a good decade ago.
Adele was legitimate however others have pretended to have an Adele grade story. Lily Allen also claimed to have been discovered on mySpace but her dad was rock and roll celebrity so her efforts can be dismissed as nepotism. Often these links to rock and roll celebrity are not obvious because performers use stage names that sound like real names. Their children don't use their parents fake surnames.
Being really good and self publishing helps you build your resume in a superstar industry, but it's nearly impossible to actually achieve commercial success without a well-established sponsor. Adele probably wouldn't have gotten far on her own if XL or some other label hadn't picked her up. Her accomplishment before then was a really good resume.
I don't think this phenomenon is limited to one form of media. "The Martian" by Andy Weir was originally published on his website one chapter at a time. The internet has, in general, lowered the bar to publishing any kind of work significantly - be it games on Steam, songs on YouTube, or ebooks on Amazon. There are now new avenues into the wider industry, but the big players are still the big players.
