Because it wouldn't be as directional as a big dish. The relative size of the dish vs wavelength plays a factor here. If you make a dish thats the size of the wavelength it wouldn't even work as a dish, it would receive the signal from all angles. This is also why high gain antennas for higher frequencies can be much smaller than for e.g long waves.
It works two ways: one is that you can go lower in frequency (e.g. bigger wavelength) if your dish is bigger, the other is that you just capture more of the signal. If you take light as a metaphor, a bigger lens will just provide more 'signal' at the focal point.
