It's because a relative clause must be used to introduce the verb.
From Gramadach na Gaeilge:
Syntax of interrogatives:
Interrogatives contain for the most part some (invisible) form of the copula or, better said, the copula is included in the formation of the interrogative. Interrogative particle and intentional copula comprise the copular clause.
If a verb should be incorporated (e.g. "who says that?"), there is still the need of a real subject, which is then replaced by a relative clause with the verb: e.g. cé a deir sin? = who says that?, lit.: "who is(it), that says that?". cé a rinne sin? = who did that?
Unlike some other European languages, Irish and English both clearly differentiate between the simple present ("eats"/itheann) and the continuous/ progressive present ("is eating"/tá ... ag ithe).
Cé a itheann sicín?/"Who eats chicken?" and
Cé atá ag ithe sicín?/"Who is eating chicken?" are not equivalent.
Similarly, there is no plurality in "who eats chicken?" - the "s" in "eats" is not a plural marker, it is used in the 3rd person singular - "he eats chicken", and not used in the plural "we/you/they eat".