Because in English "twelve thousand" is not interpreted as 12 occurrences of 1000, as in 12 cars, but rather as one occurrence of 12000. Why it's like this, is a matter of speculation.
Languages are different and I guess some languages view 200 as a plural, as two occurrences of 100. English, Dutch and (if I'm not mistaken) FrenchDutch do not, and French, as 200_success points out, swings both ways: 200 being deux cents, but 201 being deux cent un.