Same reason as in all our 400 billion star systems there arent any protosystems, its a limitation of the otherwise admittedly brilliant stellar forge engine.
There are so many systems out there just waiting for a proper graphical overhaul. I mean, technically any T-Tauri or Herbig stars you come across with rings around them (they are rare, but a pretty cool find) would be the exact candidates for being properly turned into proto-systems. After all, the star is already the right type, zero planets and a large rocky-metallic ring system. It doesn't take much to push that into being a proto-planetary disc, I reckon.
So, the skeleton of that content is there, it's just not properly dressed yet.
Just like stars need a graphical overhaul to give them more personality. Sure you can be wowed by really massive stars that you come across, or by suddenly jumping upon a star class you haven't seen for a while, but when it comes down to it, all the stars are just big spheres with slightly different colouration and the same repeating set of plasma animations. All of that is done scientifically and accurately for the most part, but there needs to be some more variance in the look of stars to really make them interesting finds - some with huge envelopes, some spinning rapidly, some massive stars where there's no way of telling where the surface is, some that are extremely oblate etc... as well as giving them dangerous flares and prominences, or using some of those lovely volumetric effects to engulf ships in thick plasma fog to make you feel like you're deep diving into the star, and so on...
There's so much that could be done to liven up the galaxy in general that just involves improving what's there at the moment.