An illustrator can adopt techniques used in fine art divisions to illustrate some concept or promo art for a game, or make textures (which, yes, are usually drawn/painted) for ingame graphics. That's just a tiny example.
Art is just an expression of life. Stick "fine" in front of it, and it suddenly becomes some blueblood's investment property, hahaha
People overthink art, especially when you stick "fine" in front of it. I don't know wether to blame the "fine" artists themselves or the layman for doing that.