Because sears are hardened, and thus brittle, metal and they are breakable. All it takes is a sharp blow to the tip of the hammer, like dropping the gun. And once the sear breaks, nothing stops the hammer from falling. There's also the problem of a worn sear or notch - it is possible for a hammer to slip past the half cock notch if the sear and/or the notch are badly worn, a condition the owner would not necessarily be aware of.
Not all guns are nice and new from the factory. Assuming all guns work like they did when they were new is not a good idea. The half cock position is not a safety. It's better than full cock or hammer down on a loaded chamber, but not as good as hammer down on an empty chamber. And most folks prefer a full measure of safety over a partial measure. Not all, sadly, but most.