Because its simply dehydrated coffee.
You can make it at home...just brew some coffee and let it evaporate. The remaining crystals are your basic instant coffee.
Commercially, both regular instant and freeze-dried start out this way, though I gather that its brewed till the coffee is ashen in colour and totally extracted. Once most of the water has evaporated its sprayed into a silo of hot air and that evaporates the remaining water, and by the time it gets to the bottom youve got instant.
Freeze-dried is a little more complicated - the evaporated coffee is frozen at very low temps, then the remaining ice crystals are removed.
Doesnt matter if its robusta or arabica - havent had a good one yet. Though seems like the brands are pretty well labelled.