Because it's not the correct English. In English you need to use "there is/there are".
That's just not true. It's the same grammar as "How many days are in a week," or "How many apples are on the table?" Just because constructions using "there" are valid doesn't mean it's required.
Just so you know: כמה -ערים- יש בישראל Sounds simmiliar to the phrase כמה -הרים- יש בישראל
The only diffrance is that ערים is cities And הרים is mountains
בהצלחה! = Good luck!