Former UI guy at Google here.

The explanation for why they do it is pretty simple: localization hinting. From country to country, the text will change but the icon pictures won't. So if you find some how-tos or guidance online that has screenshots but wasn't made in your language, you can still follow along by lining up the icons.

There are other reasons too but that's a big one.

You don’t have to have icons for every menu item for this to work though. One copy icon is enough for the whole block.