Because sometimes they come in matched sets.
Peeve One: Criteria is a plural, dammit. The singular is criterion. (The word is Greek in origin – the –on ending is a clue.)
Peeve Two: Data is also a plural. The singular is datum. (This one is Latin. Again, the ending is a clue – the –um is a neuter singular ending.)
Both of these words are, as is the way of language, in the process of slowly morphing into singular nouns, but they aren’t there yet, and until they get there, some people, like me, are going to bristle up at the usage.
(In principle, I’m a descriptivist. In practice . . . well, in practice, I get peevish sometimes.)