The motion of the Earth in space is very well determined and studied; the thermal expansion and deformation of the structure is most likely the dominant source of uncertainty for the pointing.
But also the position of the sun in the sky is not predictable enough to do this completely open loop because of variances in atmospheric refraction... but it's pretty easily measured at the site.
I think it's a scale issue, each mirror needs the same apparatus and AI, but doing it traditionally with reactive optics would need a multitude of different pieces?