Your curves don’t touch each-other! if you zoom in enough you’ll see it directly on screen.
Almost certainly because the original solid geometry has gaps, maybe it was made with low accuracy or the exportation to rhino lost some details.
On rhino, if you use the command “Join” and select the curves 1 by 1, rhino will trigger a message and ask if you want to join even with large gap. (If you select all them at once it will just silently fail).
I’ve made a simple c# join curve with custom tolerance:
private void RunScript(List<Curve> C, double t, ref object A)
{
A = Rhino.Geometry.Curve.JoinCurves(C, t);
}
join curves tolerance.gh (15.7 KB)
Edit: i guess, if you explode your “closed” solid and then launch “Rebuild edges” command, you won’t be able to join it back again in a closed polysurface.