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.
It sounds very easy when you say it. But trust me, flipping a house is not as easy as it sounds. The pricing of the house is never stable. Yeah, in the long run, it does go up, but so does inflation. So if you are willing to make a good profit, you need to be waiting for the right time. So you don't actually get money instantly to buy/build another property (by erect, I am assuming that is what you said). There might be also a case where you bought the house during a housing bubble (like right now). If you aren't able to sell it at the right time, very highly likely you will be losing a lot of money when the bubble bursts. On other hand renting is a continuos source of income/money for you. People will never stop renting and it is very unlikely that a rent goes down. Rent out few apartments/buildings, you will be set for life.