[genocidicbunny]

I think there's both a practical and aesthetic reason.

The practical reason is that people have different sized pots and pans. I don't want to put my small sauce pan on a large burner that will waste the majority of its heating surface. Similarly, I need a large surface to heat a big pan.

The aesthetic is that a lot of cooktops are combination stove/oven and thus have to be big enough for the oven bit. Once you already have that surface area, you might as well fill it up to make it seem more substantial. A stovetop with room for 4/5 burners will look like the manufacturer skimped out if it has only 2 burners.