Because contaminants can get past the seals, as well as air getting in the system. A full brake bleed is a good idea from time to time to flush the air and contaminants out. Think about everytime you use your brakes, you pull the lever and there's a bore piston that pushes the fluid from lever to caliper. The calipers have 2-4 pistons (depending on your brake model) that move each time you use the brakes. The pads wear, the caliper pistons stay out a little more each time, the lever starts to feel mushy because you have the same amount of fluid in there but more space for the fluid to have to fill behind the pistons to move them.
Topping up fluid is ok for a quick fix, but if you're changing pads, or it's been a while and the brakes feel like shit, a full bleed should be done.