Could you please list which airports you've been to have this, and where it is located in them?
It's not that I doubt airports have staff canteens, it's more that I have trouble believing anyone can just waltz into them. And a casual web search brings up only results about one airport actually having one open to the public.
But I don’t remember having trouble doing this in Europe or Canada.
It’s not publicly advertised because airports prefer you go to the overpriced airside concessions they can charge obscene rents for, and airport workers just know where it is; they don’t look it up.
Next time you fly, ask a janitor or other airport employee (the ones in the shops don’t always know and/or eat at the concessions because they have discounts) and see for yourself. It was typically ground staff, janitors, security guards, baggage throwers, public transit employees eating there.