There are certain words that technically have a kanji spelling, but are usually writen in kana even outside of signage. I think it's mostly words that would have been important to historically less-educated people, so a lot of them are foods. It's probably not a coincidence that it's not uncommon for women's names to be written wholly or partially in kana (e.g. かおり, このみ), who traditionally are the ones to buy the groceries.

On top of that, for signage specifically it's lot easier to read things like そば, ラーメン and たこやき/たこ焼き from a distance than 蕎麦, 拉麺 and 蛸焼き.