I've never seen instant coffees being sold consisting of varying roasts (i.e. light/mild, medium, dark etc.) does such a thing exist? Or is there a reason why they haven't been tried?
There is no reason why instant coffee couldn't be roasted differently. It's made from real coffee beans after they have been roasted and ground. Instant coffee is just dehydrated coffee basically.
I think the reason you have never seen different roasts types is because instant coffee is made more for convenience. It's supposed to be quick, easy coffee. It's made for the "Amateur coffee drinker"(If there is such a thing). For that reason those purchasing the coffee probably don't care much about how it will taste and therefor care very little about the roast type.
Of Course one can. Instant coffee is made by a process called freeze drying or lyophilizing ground coffee beans but since it is supposed to be quick and instant I think people who manufacture these do not bother with marketing the type of roast on the bottle. It is simply a matter of branding really.... for a person who does not care much other than the time and effort to make coffee surely wouldn't bother with the type of roast.
Although a company called BRU did manufacture instant coffee's from different regions around the world specifying strengths and aromas. you might wanna take a look at that. It's called 'Bru Exotica'