Focusing on flipping of houses would be more profitable than waiting on rent. Actually, selling houses is a business, and shouldn't be a one time thing, the money generated from selling one property, can erect another one, which can still be sold. All the same, a rented house wouldn't raise enough money to build its replica in a short time. Therefore, the owner is not interested in selling houses, but wants a stream of income that doesn't ask for his time anymore.
you are right, that selling a house or apartment will get a lot of profit and more instant than renting it. However, sometimes people who rent their houses do not want to sell their houses. they choose to live in a new place, but still want their old house as an asset and investment. So, what they do is rent out the house they own. it will have at least two advantages, the first is that their house may still be maintained by someone else, the second is that the assets they own will have a high price every year.