It's because "pants" always have to be plural in English. ("A pant" doesn't exist.)
In india, we usually say it as a pant(even in a non-english conversation), but yeah.. then we don't speak english correctly all the time.. Thanks for the correction!
Sorry probably being stupid but can I check that pantolon means trousers not pants as I would only use pants for underwear but I know in America they call trousers pants thanks