I should double-check this, but I think there's also a boring historical reason: back in the day, Greek had a word that meant "making marks on a surface in order to convey meaning" (what we'd translate as 'write,' 'draw' or 'paint') but didn't have a separate word meaning "smearing paint on a surface in order to depict an image" (what we'd translate as 'paint'), or 'making marks on a surface with a non-paint medium in order to depict an image' (what we'd call 'draw'). And then when later-on Greek picked up those new words, it kept the old word to describe the production if icons. And so it's 'writing' an icon, not 'painting,' simply because back-in-the-day Greek didn't have a separate word for 'paint'.