You certainly could, and it's a method sometimes used for NVIS purposes.
The issue for general use is it really only helps add gain going straight up, like a 2 element beam pointed vertically into the sky. That's not helpful for most HF skip propagation where you want your signal to go out at lower take-off angles.
ETA, here's a model of a 40M dipole at 65 ft, which would be about a half-wavelength above ground (ideal). You can see the gain and take-off is totally unaffected by a same sized radial/reflector on the ground (I modeled it here with and without the reflector, results were identical).
Attached FileThe reflector won't start having an appreciable effect until the dipole is much lower to the ground, and at that point you'll have most of your signal going off at high angles (and mostly out to space) anyway so reflecting more to the higher angle isn't helping too much. But again, in some cases for trying for NVIS or shorter skip, it can give a bit of gain there, band choice and propagation dependent of course.