If you want a historical explanation, it is thought that Proto-Indo-European, often regarded as the ancestor of most languages between Iceland and Calcutta, had three grammatical numbers:
- Singular : 1
- Dual : 2
- Plural : 3,4,5...
(Note that other things that we now count as numbers, such as 0, 2½, -4, π are modern inventions.)
Most languages have mostly lost the dual but many languages have tiny remnants. Modern English has a few words with a b in, such as both, a word that does not exist in Gaelic or French. Old English has words for 'we two' and 'you two'. Middle Welsh had special forms for things that you usually have two of, such as eyes. This is a part of a general discussion on Wikipedia Dual (Grammatical number).
Old Irish had separate (and complicated) forms for the dual, but this has simplified to just using what looks like the singular, and only after dà (and remember this causes lenition). The link shows it is not quite as simple in Irish as it is in Gaelic, but that is not our problem.