User guide and WASM demos for nphysics: Rust physics engine with multibodies

Well most physics engine, including nphysics, bullet, box2d, are able to achieve much more realistic simulations: just increase the number of iterations for the constraints solvers, and reduce the timesteps length. Of course this will no longer be real-time but will be suitable for generating animations. In the end, it always boils down to a compromise between performances and realism unfortunately.

Because people build these projects in their spare time.

Just like you have observed there's no perceived business value to managers yet in WebAssembly etc. for companies yet that aren't directly involved with 3D (like AutoCAD Web, game engines, ...) or other visual rendering (like PDFKit), so most of the things that are being created are just cognitively stimulating for the developers creating them, and that's not necessarily creating UIs.

Building things like this physics engine is like playing Lego for adults.

When you create UI, people always have opinions and you suddenly need to justify why things work differently than what they think is how things should work. That's exhausting.

You could make a startup out of it..

Wait for the spec to mature a bit more, and for better tooling to emerge, and even then chances are the solution will be a port of QT.