That's because Hindi doesn't have an indefinite pronoun like 'everyone' that is singular but refers to a group of people. I suppose you could literally translate the Hindi sentence into 'All are in Delhi' though it sounds weird.
Edit: Came across my comment again and realised Hindi does have such a pronoun. It would be हर कोई.
हर कोई दिल्ली में है। - Everyone is in Delhi