Because unlike English, Hungarian uses the singular to refer to things that come in pairs.
- Elhagytam a zoknimat. -- I lost my socks. Both of them.
- Elhagytam egy fél pár zoknit. -- I lost one sock. Literally: I lost half a pair of socks.
In Hungarian it's incorrect to use the plural when it's only one pair you're talking about. For instance you can't say (well, you can, most people do not care, but it's still incorrect) "fáznak a füleim". The correct form is "fázik a fülem", even if most likely both of your ears are cold.
But it's true that the sentence above can be translated to English using the singular, so your version should be accepted as well.
Very useful detail, thank you! Simply the fact that one must say egy fél pár zokni to indicate a single "sock" makes it clear that the dictionaries I have to hand are apparently deficient -- zokni cannot indicate a single sock, cipő cannot indicate a single shoe, etc., but only a pair of socks, shoes, etc.
From that logic, if one says négy cipő in Hungarian, would that mean "four pairs of shoes" → "eight shoes" in English?
I think once you start dealing with numbers greater than one (pair), you better specify "pár", to avoid confusion.
"Van egy cipőm" - usually means a pair of shoes. If not, you clarify.
"Van négy pár cipőm" - is the usual way to say four pairs of shoes.
For four pieces of shoes, you could say "négy darab cipő", "négy páratlan cipő", "négy cipő, aminek nincs párja", etc.
The point is, you clarify the ambiguous situation.