There plenty of things that won’t make a noticeable difference in the output, especially in libraries.

Let’s suppose I make a slight more efficient implementation of green threads, for example. I do not see how that would affect the output in a way that would be obvious, even if the library is non-trivial. Even if I slapped it with a GPL, I don’t see how I would realistically be able to check if they broke the license without first auditing the code, which I couldn’t do without a discovery request, which I likely wouldn’t have grounds for even if I could afford the lawyers for a lawsuit.