Question

why the house in Rajasthan are constructed with thick walls and flat roofs

Answer 1

because there is too hot

Answer 2

this is because Flat roofs are efficient in that climate. Rajasthan being a very hot place, people build houses with thick walls that are hard for heat to penetrate. While that is durin the day, these walls also help in heat loss during the night, when Rajasthan climate suddenly becomes cold.