Your car is best heated from the floor vents, and best cooled from the face vents. It’s more common to use the defrost vents during cold weather that requires heat than hot weather that requires air conditioning, and so this is the natural split for defrost mode. My automatic climate control automatically diverts between floor and face (and if heating, a mixture) based on the temperature it’s trying to achieve, but it still limits me to the same split choices when it comes to defrost.
Cost is certainly an option, too. There are internal flaps that direct the air to the appropriate vent, which are either servomotor driven (rare) or pneumatically driven (common, even on high-end vehicles). In some cases, the flaps are interlocked, or even more often, the flap isn’t just a door (permitting/blocking flow) but a diverter (choosing where the flow goes). So, the more flaps there are, the more pnuematic lines you have to squeeze in, you’d have to use more doors rather than diverters, and overall size of the distribution box grows, all of which contribute to cost and weight.
All those changes aren’t generally worth it in order to blow cold, air-conditioned air into your face in the middle of winter (assuming your engine isn’t warm yet).