Well… it isn’t! The same is aid of purgatory and some other Catholic terms. Some non-Catholic Christians see the absence of these words as proof that they are unbiblical.
If we apply this logic to the rest of our Christian vocabulary, we’ll have to denounce the incarnation and the Trinity as unbiblical as well. If we throw out those two words... well…we no longer have Christianity, do we?
There is ample evidence for the papacy, purgatory, the incarnation, and the Trinity in the Bible, even if the words don’t appear there. The words have been picked after the writing of the Bible, but the truth those words reveal is still there. So don’t worry about whether or not a word exists in the Bible; look for the truth that word reveals.