Scroll far enough on any platform and the same few pieces of audio recur under completely unrelated videos. The convergence is produced by three separate systems pushing in the same direction.
The licensed library is small by design
Platforms cannot let users attach any recording to any video without legal exposure. They solve this by maintaining a cleared library and steering everyone into it.
That library is large in absolute terms and tiny next to recorded music as a whole. Everything outside it carries the risk of muting or removal.
Creators are rational about risk. Given a safe pool and an unsafe one, almost all of them stay inside the pool, and the pool is shared by everyone.
Sounds are a discovery surface
Audio is not just decoration; it is a category. A video carrying a given sound can be surfaced to people browsing that sound, which is a route to an audience that does not depend on followers.
Using an already popular sound therefore attaches a new video to existing traffic. Using an obscure one forfeits that entirely.
The incentive is unambiguous and it compounds. Popularity makes a sound more useful, which makes it more popular.
Ranking rewards familiarity over novelty
Measurement favours posts that hold attention in the first seconds, and a familiar sound signals to a viewer within a beat what kind of video this is.
That recognition buys the crucial extra second. An unfamiliar track asks the viewer to work out the register of the video before deciding to stay.
Over millions of posts this small advantage becomes a strong sorting force, and the surviving sounds are the ones that are already known.
Material crosses platforms with its audio attached
Videos are downloaded and reposted between services constantly, and the sound travels inside the file. A track that becomes common on one platform arrives on the next fully formed.
The receiving platform's own library then absorbs it, because users request it and rights holders want it available. Convergence completes itself.
This is why the sonic character of unrelated apps is so similar despite their different origins and audiences.
What this does to the music itself
Tracks are increasingly edited into short usable segments, and the segment that circulates may not resemble the full recording. A song can be widely known through a fragment that is not its chorus.
Rights holders respond by preparing cut-down versions in advance, which is a real change in how recordings are delivered.
The music adapts to the container, as it has done before with radio edits and album sides. The container is simply much shorter now.