It’s a bit like asking, Why aren’t the continents moving? or Why don’t the constellations change? In these cases the time scale is so slow that it’s difficult or impossible to directly see change over the span of a human lifetime (and in many cases, even over recorded history). As a historical science, most evidence is indirect or inferred. Yet a consistent picture can be built.
Here’s where I wish I had a better linguistics background. I think the emergence of a “new language” (not just a regional dialect, an argot, etc.) is a complex social and cultural phenomenon that plays out over many generations, even when it occurs relatively rapidly. Not counting computer languages! There’s an obvious analogy to evolutionary biology, where new species emerge over time that are distinct from their ancestors. Evolution involves changes, not to individuals, but to an entire interbreeding gene pool.
I don’t know how far to push the analogy, but my not-so-educated guess is that sounds and words and grammar are linguistically analogous to a gene pool (meme pool?), accruing change slowly until some kind of non-linear tipping point is reached, then a new language emerges over a few generations. Not sure if new language “speciation” is a continuous process versus punctuated equilibrium. where have I seen that before? ~
Because oral language is only 50–100,000 years old, and writing only 5–10,000 years old, I’d also guess that linguists don’t have the wealth of evidence that evolutionary biologists are accustomed to for constructing lineages. Extinct languages may leave fossils, so to speak, but I think it takes a lot of linguistic analysis to find them.
I’d be fascinated to hear details that are known by linguists & to confirm my hunches?. There’s a book called The Emergence of Pidgin and Creole Languages still on my Amazon wish-list.
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[addendum] @Zen: I just realized the key word is simple. So maybe Esperanto is closest to what you seek for some kind of simplified world-speak universal language. So far it’s been a flop, even in countries that use the Roman alphabet.