I have seen many recipes for cakes, cupcakes, coffee buttercream, etc. that call for instant coffee than the freshly ground/brewed coffee because the instant coffee produces much better flavour. Why is that when fresh coffee is considered much better and flavoursome than instant coffee? 
Same reason that recipes call for cocoa powder and not a cup of hot chocolate - it doesn't contain any water, and it's easy to control quantities.  If you used brewed coffee then the recipe would have to be adjusted for water content - assuming that's even possible and you're not adding the instant coffee to other dry ingredients. There may not even be enough water in the recipe to make the adjustment, and if there is, variations on coffee beans/grounds, brewing methods, etc. would make it impractical to estimate the quantity needed.  That's not to say you can't use brewed coffee if you want to, but as a recipe writer it's far simpler to work with fixed/standardized quantities, and brewed coffee is about as far from that as you can possibly get. 