> I've speculated that if someone has a hard time expressing themselves to other humans verbally or in writing

I don't think they have actually problems with expressing themselves, code is also just a language with a very formal grammar and if you use that approach to structure your prose, it's also understandable. The struggle is more to mentally encode non-technical domain knowledge, like office politics or emotions.

That's true. But people have had formal language for millennia, so why don't we use it?

Here's my hunch. Formal specifiation is so inefficient that cynics suspect it of being a form of obstructionism, while pragmatic people realize that they can solve a problem themselves, quicker than they can specify their requirements.

> But people have had formal language for millennia, so why don't we use it?

In case you don't refer to the mathematical notion of formal, then we use formal language all the time. Every subject has its formal terms, contracts are all written in a formal way, specifications use formal language. Anything that really matters or is read by a large audience is written in formal language.