Most things that spread widely online are not individual pieces of content but formats that other people can reproduce. The reproducibility is the mechanism rather than a side effect.
Participation is cheaper than creation
Making something original requires an idea, and ideas are the scarce input for almost everyone.
A template removes that requirement, supplying the structure and leaving only the details to fill in.
The cost of taking part falls to a few minutes, and anything that cheap will be attempted by an enormous number of people.
It removes the risk as well as the effort. A version that fails is understood as a poor attempt at a known format rather than as a bad idea belonging to the person who posted it.
The format must still allow a personal detail
A pure copy is uninteresting to post, because it says nothing about the person posting it.
Successful templates leave a slot for something specific: a job, a family situation, a local detail that only the poster can supply.
That combination of fixed frame and variable content is what makes each instance both recognisable and worth watching.
Formats that leave too much open fail for the opposite reason, because filling a blank is the work the template was supposed to remove.
Recognition is what carries it
A viewer who has seen the format before understands the joke immediately and can appreciate the variation rather than decoding the structure.
This makes each subsequent instance easier to consume than the last, which is the opposite of how repetition usually works.
It also creates an incentive to watch, since the pleasure lies in comparing versions rather than in any single one.
Audio and editing tools do the enforcement
Platforms that let a user attach an existing sound or duplicate an editing pattern turn a format into a button.
Everything using that sound is grouped together, so participating also means joining a visible collection.
The tooling therefore determines which trends are possible, and formats requiring capabilities the app does not provide simply do not spread.
Saturation ends it rather than boredom
A template dies when the variations run out, because every remaining slot has already been filled by someone.
The final stage is usually parody of the format itself, which signals that the structure has become more familiar than any use of it.
Formats capable of many variations last months, and narrow ones are exhausted in days, which is why the lifespan of a trend is set at the moment its structure is defined.