Constructing a spacecraft capable of sustaining a human being for multiple years, landing them with all necessary support equipment, building an ISRU plant for which solid designs do not currently exist, and for which even the principles are still very much the subject of research, launching and returning is much, much more complicated than sending an unmanned spacecraft or two to recover a sample. This is really not too far off from what JAXA is already doing with Hayabusa 2.