I mean, there's a quite recent one. Here, take a look :https://beamng.com/resources/demoli-tron.7123/
I think that it takes quite bit of time to plan and build a good layout, also you need to plan ahead how you build instead of freely drag pieces around, so that might limit number of people that has dedication to complete a track. I would like to build few tracks, but I'm having slight time issues right now myself, so that taking 4 hours or more to build a track is not happening right now.
Might just be me, but another reason might be the negative thoughts against Trackbuilder tracks, as it still takes a bit to set it up and the effort doing it may not be worth it with the hate it might get. Personally, I like them, though don't play/make them because it takes a bit to spawn them.
Ever heard of "lack of interest"? That's the case here, most BeamNG players lack interest in making track builder mods. That's as clear as I can answer as of now.
I don't release mine but I play with it often And I think that this goes for the majority of people that use the trackbuilder. It is fun to play with for yourself, not so much to pack as a mod and release it as such.
Based on my experience, people have nothing to say about trackbuilder tracks, so I'm not sure that it would be so bad to release yours, it looks fun, see nobody has said what is wrong with my track: https://www.beamng.com/resources/racetrack-one.6504/ It is 3rd most subscribed on track builder mods section, but maybe it just is not enough good. There are only 11 tracks on trackbuilder mods section, even sharing track is quite simple, you just press two buttons and then browse where your mods are. I think that easy way to share times would be helpful though. For example RBR Kegetys made program that wrote formatted HTML page from your best times, then people put those up and could compete. I think that gametimes online had also leaderboards for that game, which did make people to play much more of those stages. BeamNG has highscores.json for a skilled that has time, it would not be big task to get that to html page or something. Problem is time or skill, or both, maybe developers will add some time sharing functionality at some point, I'm afraid without it at this rate this nice feature falls flat.