One reason is simply that languages change over time, especially with relation to its culture. Meaning as a groups culture changes so does their language. In general languages having a notable change takes a few hundred years so it is a slow process.

So even if you want to assume there was a “first” language (which I don’t think is correct), over the thousands of years of human migration, our cultures have changed very drastically from one another and so do our languages.

There are many answers to this.

Languages change over time. Even if you started with one language, as people spread out and different communities formed, those dialects would eventually change enough to no longer be mutually intelligible with the original language. And related to your wording, we are indeed using “flapping meat” to communicate, and while it can produce and reproduce sounds with regular characteristics, these sounds are not completely and exactly the same between people or even between utterances. Our perception of what forms a given phoneme can encompass a range of patterns that all “fit” to an archetype for what we expect for that sound. Some sounds have enough characteristics of another sound that we categorize them as being the same one, even if there are actually greater differences in their production than between two other sounds we recognize as distinct.

So languages naturally change over time, and when we produce sounds we are not doing so with perfect fidelity and precision each time. Beyond that, we must remember language is a system of communication that communicates more than just information. A crucial use of language is strengthening social ties between individuals or groups. Groups may actively speak in a certain manner to identify a person as a member of that group, which can be another driver of language change over time. It also means that we don’t necessarily care about how efficient we are when using language, because efficiency frequently isn’t the point. We are complex social creatures, and our ways of engaging in complex social situations are similarly complex, with human language being no exception.

Language evolves, and it does so at a much faster rate than biological evolution since it is driven socioculturally, even though some of the same factors that can drive biological evolution can also be at play (geographic isolation, reproductive isolation, sexual selection even). With this in mind, realize that humans have been speaking, joking, telling stories, and all around using language for hundreds of thousands of years. Populations of humans spread to every corner of the globe and innovated new societies, new ways of living, encountered new environments. Why would they also not use new words, new grammatical structures, as they were confronted with new physical environments and social concepts?

I’m sorry I can’t give you a short answer, because there isn’t one single simple short answer. I also didn’t want to just drop an article about the origins of language, because that is a frequent subject of debate among linguists and anthropologists. But I hope my answer helped. Source: PhD in linguistics.