Yeah this didn't work well at all and is no longer being done. Lasted like a week.

I have modded and built levels for games in the past (~Quake 3 era). The tools provided by the developers are usually powerful and awesome starting points, but they aren't idiot proof and they aren't "user friendly" when used in the context of the average "omgz I liek maeking gamez!!111" individual.
Things have changed a bit with Steam integration and whatnot, but the 'good' mods aren't built with 3 or 4 clicks in a rigid mod-maker app, and there is still challenge and effort required by the end user to achieve anything resembling a fully featured modification. There's literally no reason Stern or JJP to take on enabling people in this way imo - even when ignoring the idea of burning your house down accidentally - there's no money to be made, with enormous cost and liability to enable it.

Also the equivalent of your Steam Workshop example already essentially exists in Visual Pinball and Future Pinball.