Why do baking recipes call for instant coffee instead of fresh ground 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.

Fresh ground coffee requires some sort of brewing process to extract the flavor, generally extended time in hot water. If you just dump some into a cup of warm water, you won't get much out of it - some wet grounds and some slightly coffee-ish water. Same goes for baked goods: coffee grounds won't efficiently release their flavor. If you brew it first, you'll have to add a lot of liquid in order to get enough coffee flavor, likely more than the recipe called for in the first place.

Instant coffee, on the other hand, is designed to dissolve and release all the flavor. Used in baked goods, the same thing happens. Sure, the coffee flavor itself isn't as good as you'd get from real coffee, but you get all of it. It also happens to be much easier!

The main alternative is to use espresso; the flavor is much more concentrated, so a recipe may be able to replace some of its liquid with espresso and get enough flavor out of it, without throwing off the recipe.

For most baking applications, you want something with concentrated flavor that adds minimal moisture and won't change the texture of the finished product. This usually means adding either a coffee extract or instant coffee. For instance, in the cakes and cupcakes, adding brewed coffee would add water and run the risk of making the cake tough, while grounds would make the cake gritty (and probably not actually add that much flavor). In buttercream, adding brewed coffee would involve too much liquid, probably make your frosting separate, and again, grounds would make the frosting gritty.

While fresh coffee has a much better flavor for the purposes of drinking, in most baking applications there are enough other ingredients adding richness, sweetness, etc, that only the base flavor of the coffee will come through. Any more nuanced flavors from good fresh coffee would be lost anyway.

When I make coffee to substitute for instant I grind it real fine and make it in a small french press using triple the amount of coffee I would normally use for drinking. I strain it and I reduce the amount of liquids in my recipes. There are things it won't work for such as in chocolates as water will ruin chocolate. I haven't tried brewing it with heated cream. I find I need no more then two tablespoons of the triple strength french pressed coffee to substitute for 1 tsp instant.