Maybe because it takes most people hundreds or thousands of hours to learn a natural language, but the average programmer can get a basic understanding of a new programming language over a weekend? Plus, there's arguably no advantage of one natural language over another, whereas different programming languages can make very different tradeoffs.

Humans started out with only one very limited language, then spread out to cover the world, and new languages flourished. As technology progressed and the world became smaller, having fewer languages (or at least, a few common languages) made more sense. With the advent of the internet, the world has become so small a single language might be the best option.

Programming started out with one very limited language (machine code), the spread out to cover the world, and new languages flourished. As technology progresses and the world becomes smaller, having fewer languages (or at least, a few common languages) will make more sense. With the advent of the singularity, the world will have become so small a single language might be the best option.=