My panniers are waterproof, and they do get wet. (There's not really room for the six panniers we carry between us in our little porch). That's not the point.
With two of us in the tent, there isn't room inside it for anything much except us. We don't bring our shoes in, either, or it quickly becomes muddy or grassy or dusty or sandy inside. We don't cook in the porch (not with a petrol stove!). It's just a place to park shoes, waterproofs, handlebar bags, mugs, food, pans and all that sort of stuff, and the bit of groundsheet there means it doesn't all get damp.
The groundsheet itself becomes damp, sure, but it doesn't soak up any water, so a quick shake and it's ready to pack - in the "wet" pannier along with the tent, stove, pans and any wet clothes.
If you only camp when it's dry, on short, clean grass, then I agree you'd have less need for something on the ground on the porch.