I've had one snap on me.
It's not solid metal, per se, but rather sintered metal. Google will give you the process, but basically sintering is taking finely powered metals and fusing them together with pressure or heat, or often both, to the point where they do become one solid piece of metal.
The are many advantages to sintering including controlling the content and purity of the metal. One big disadvantage is that if there is any impurities, the finished metal will be weak and brittle at the impurity. A wiper arm isn't exactly a mission critical part, so the purity sometimes isn't what it might otherwise by. Just one or two molecules of an impurity is all it takes.
So take comfort in that when you realize that the majority of the internal parts of your drive train, like all of the cogs and gears, are composed of sintered metal.