[Spivak]

Because single humans are busy, get bored, get distracted, stop wanting to maintain servers, stop wanting to pay for servers etc..

If the product was released by some self-sustaining entity like a business, foundation or otherwise bankrolled and staffed by parties with an interest in keeping it up then it would be much less of an issue.

The value in almost all software is the maintenance. React being good software is one thing, but what makes it so attractive is that Facebook and (and these days many others) have a financial interest in keeping it maintained.

[Crinus]

Yeah, because companies never abandon popular software (that might be Google's motto) and there are no "single humans" who work in projects for years.