Because this is largely outside the scope of most 3D game engines. Rendering crisp 2D lines (or any anti-aliased complex vector polygon) on the GPU is hard: it involves a lot of high level decisions about how to process, structure & submit the data to the graphics pipeline. Depending on the application, one approach may be vastly superior to the other.

I do think ThreeJS should include basic thick screen space line support, though, as it covers most use cases and doesn’t add a lot of bloat/complexity to the core framework.

I think it's because nice lines aren't a super big use case for WebGL. Most people use it for 3d scenes, 3d objects, etc.

In other words, I suppose that three.js doesn't have nice lines because nobody contributed it, which is usually a sign of low-ish demand. (I know nothing about three.js governance and goals so I might be mistaken)