The problem imho is bandwidth. Unless you are iq or similar, creating geometry and textures from thin-air with pure math is just not possible, so you need to load 3d models, which are much heavier than their html/png counterparts.

For individual models, like if you browse sketchfab, it's not such a big deal if the artist optimized things to be "game-ready", but for full environments, it's a problem, especially as audience expectations are influenced by what they see with multi-gigabyte assets on standalone games and simulations.

I don't think this is a complete showstopper, like 10-20 years ago we had the same problem with 2d games and Flash, but as long as the experience was compelling enough and the preloader wasn't completely boring, people waited, but there has been a shift towards wanting things to be instantaneous on the web (e.g. video with adaptive bitrate switching, blurry but instant > sharp but buffered)

Anyway, I guess my point is I don't think it's exactly a technical limitation of programming, as much as a conflict between audience expectations and wait time