Why does everyone forget Sharp Ends, or call it "optional"?

Obviously not everyone, but I see this constantly when recommending the order to new readers and it makes no sense to me.

Every book is "optional". But if you've opted to read "The First Law" series by Joe Abercrombie, why would you be like 'nah, you can skip that one'?

Do you guys also recommend skipping chapters in the main books?

I mean, you could probably just skip the first 3/4 of "Casualties" if you want, it's all side characters and kind of a distraction from the MAIN PLOT ;)