# Why are loop antennas placed horizontally in HF

I am thinking of a large (circumference $\approx \lambda$) loop antenna at low (HF) frequencies. At higher frequencies the loops are used as quad antennas that have relatively high gain towards the axis of the loop. However, at HF band these are often positioned horizontally above the ground. One would think that the ground would act as a reflector and the power would be directed upwards which is obviously undesired. Are the reasons purely mechanical or is there some other reason to use horizontal loops instead of vertical ones?