meaning - Is 足りるだけ with a negative verb idiomatic?

I've encountered sentences like:

「皆【みんな】に足【た】りるだけの椅子【いす】がありません」and

「皆に足りるだけのコーヒーがありません」.

In English, we wouldn't say, "There aren't just enough chairs," except perhaps as, "There barely aren't enough chairs," in which case we are emphasizing that we are just short of the necessary amount. In the context I encountered the foregoing uses of 「足りるだけ」, this did not seem to be the case, which made me wonder if this is just idiomatic language.

I guess my question is why is「だけ」is being used here.