But nikkkom, that makes no sense.Cost. Separating transuranics from fission products requires further processing steps.
Each additional step costs a lot when you work with very, very radioactive materials: it must be done in airtight building, all gaseous emissions and liquid effluents require elaborate filtering, all vessels and piping need to be very durable (since repairs are extremely costly, if practical at all), and all of this stuff requires heavy shielding.
1: Vitrification is not cheap; I doubt that even superficially it is significantly cheaper than reprocessing; you know perfectly well that one does not simply dunk spent fuel rods in molten glass and bobsyouruncle. You first have to get the relevant materials out of the problem material, and convert it to workable form, you then have to work out into what kind of glass (LOTS of glass, please note, it is not just one part Pu, one part glass, one eye of newt and one tongue of dog!) and how to incorporate it as a homogeneous mass. That will COST!!!! Certainly it does not cost as much as isotopic separation, but that is a separate concern; we don't have to separate isotopes until we need to harvest isotopes for purposes that will pay for the process.
2: That process itself produces waste, generally chemically dangerous and generally unacceptably radioactive as well, and not easy to vitrify. More cost!
3: Lots of very smart people have been busting guts (or cerebral vascular systems) to produce ceramic matrices that can be relied on not to dissolve or leak under various underground conditions, and considerable credibility gaps have grown concerning some at first promising-looking materials. Cracks, crystallisation, solution, you name it. So we have a nice new one that won't leak? This year? This century? This millennium? So much nicer than storage in re-usable form against the time that we could have reprocessed the cooled, unmodified fuel assemblies after the demand had grown and the techniques improved? Do tell...
4: Storage of minute volumes of UN-reprocessed high-activity waste, literally cubic metres rather than thousands of cubic metres, in a sealed chamber, accessible for extraction on demand would be a fraction of the cost of such treatment, and it might be expected to be undertaken within decades rather than centuries, after a few cycles of which it would have paid its way and and also would be much safer than the vitrified time bombs that could be expected to remain useless and unacceptably active after many millennia.
In sum, I regard the cost of a rational storage and process-on-demand as far more economical, and probably far safer than vitrification.