Nobody said that you had to tip them.
There might be something with semantics, however, between cook and chef. I’m more likely to tip a cook at a place I go to a lot. For a chef, I’d be more likely to give them a bottle of wine or something similar (which I would also do with a professional who has provided excellent service). If an accountant saved me a ton of money or helped me in a pinch, I’d have no problem giving him a bottle of wine or a gift certificate as a thank you. I know that some people just look at it as "that’s their job", but it’s nice to show people that they are appreciated, and it usually does come back to you.