Note that I do agree that ssr is a very good quality enhancing rendering technique and would love to have it on sketchfab.
As said, very complex and very heavy specially for webgl, no mipmap of framebuffer in webgl1 for example, no while loop, no VRAM estimation possible, no control on user screen size, etc. It's much more challenging than a controlled envirronment like desktop opengl4 with all latest Feature and GPU.
( Remenber that you compare desktop engine with high end GPU to webgl with web browser users... )
So technically possible and very interesting, but technically very hard to get right and fast.... whereas it would concern a very small % userbase, plus it would need more UI, and more UI that need people understanding the feature (SSR explanation not that simple, plus explain the fps tax that follows)
And there's tons of other technical things to do that ends with higher priority because not having all those downsides