The countervailing force here is the desire for command line backwards compatibility. Every time you change a default to be something that makes more sense today, you break the existing working setups of some unknown number of people...
The approach the project has settled on is to say that providing user friendly defaults should be the job of a "management layer" piece of software like libvirt, and QEMU proper should concentrate on providing flexible and orthogonal options which that other software can use to tell it what to do.