Why does the chromaticity diagram look like that?

It's probably good to start with XYZ, but we have much better colorspaces now that do a better job at correlating with our vision.

Mainly CIE 1976 L',u',v' and even more recently ICtCp from Dolby research.

CAM-16. When in doubt, ask the color scientists :)

Oklab is even better.

https://bottosson.github.io/posts/oklab/

No, it's not, by definition. It's one matrix multiplication to do an approximation of it. More here: https://news.ycombinator.com/item?id=41081832

The only claim to superiority it makes is gradients, and that's a category error: they blend polar opposite hues in the Cartesian space (i.e. x / y / z), rather than polar (i.e. h/s/l). Opposite hues mean lerp'ing in cartesian brings it through the center of the circle, 0 saturation. Thus, blue and yellow do combine to a off-white. Engineering around it indicates something fundamentally off, much less that it is better. I don't ascribe ill intent but I do worry very much about how widely this is misunderstood.