I suppose there's no reason you couldn't do that. I imagine they didn't want to do that because cables (as a platonic ideal) should just carry signals from point A to point B without fudging anything. When you start to screw with that dynamic, you end up with lots of non-standard issues that people blame on the standard (I'm reminded of contractors jamming two 100mb/s lines through a single Cat5e cable instead of just switching on a single 1000mb/s line)