Because it's not necessarily correct. There's so many fields dedicated to growing energy plants that covering just a part of those would be sufficient to electrify the entire transport sector. That's just fields for plants for Biofuels etc., not a single beautiful picturesque meadow, not a single field that grows food.

Of course covering car parks is a good idea too, but it's more expensive, and it's a climate change denier's strawman that covering fields would supposedly endanger our food supply or ruin our landscapes.