It's a vestigial structure from Bell Lab's Unix documentation which prioritizes brevity and precision. It's like mathematics on Wikipedia. It's always written by some math nerd in terms that only fellow math graduate students will understand at a minimum.

I guess in the case of Linux it also came about because the people using it and developing it were fluent in it, so man pages can follow a rigid structure with extreme amounts of jargon. It also made it easier to compress and transfer in the early internet days as a side effect?

At a time it was barely reasonable to assume anyone reading them would understand the undefined syntax they use.