This is because the two named coffee products are completely different in their qualities.
'Real' coffee is prepared relatively easy: coffee beans are roasted, sorted and ground. All the chemical constituents are safely stored in coffee cells and not released until brewed.
Instant coffee is made differently. The coffee beans are roasted and ground (same as 'real' coffee). Then, water is added to the powder and the mixture is heated under high pressure to about 175 °C. The mixture is then filtered and solubles concentration is then increased using evaporation. The resulting liquid, which is, in fact, is a very concentrated coffee drink, is dried and the instant coffee powder is produced.
So the difference is that ground coffee is actually a coarse powder from coffee beans and instant coffee - dried crystals from the liquid. When wet, instant coffee crystals reform highly concentrated coffee drink. Ground coffee particles absorb water, which enters the cells and they increase in size, acting like a sponge. You may want to make a litre of coffee from using you normal coffee maker and then boil most of the water in a saucepan. The resulting liquid will be as sticky as instant coffee (and probably have a horrible flavour) :)