When used in the singular, a countable noun is required to have a determiner.
*I bought car.
But the same countable noun is not required to have any determiner when used in the plural.
I bought cars.
Is there any linguistic reason for this?
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The English indefinite article comes from the word for "one" (Old English ān), which, because of its semantics, is somewhat resistant to pluralization. (This resistance is not perfect, however, and pluralization has happened in various cases: cf. Spanish unas personas "some people", Finnish yhdet ihmiset "some/certain people", where unos and yhdet are plural forms of the word meaning "one".)
By contrast, the English definite article comes from a demonstrative pronoun meaning "that", which can be used in the singular or plural, just like most other modifiers.
Therefore, the article-free indefinite plural (people, things, etc.) may be a leftover from the era when English had no articles (a stage that is still partly seen in Old English). Once the definite article became entrenched in English, the indefinite interpretation of article-free plurals may have become automatic (more-or-less) through the process of elimination.
The reason is efficiency. If the detectable omission of any part of speech is done consistently enough, that omission can replace a word. In this case, it is a 'zero article'. The determiner that is omitted is either 'all' or 'some', depending whether the noun is the subject or not, and whether the clause is defining or not.
It could have been done the other way, with the zero article indicating the singular; but 'a[n]' (from 'one') is so minimal already that it would be a shame to trade it for a plural determiner. The count determiner often goes away completely when a definite article is used.