Developers only provide a service of constructing a house on a plot of land. Whereas landlords actually provide the resources (capital) required to build it.
Yes, of course - but ultimately, it's the landlords who park the capital for many decades in the properties, and not the developers. So, it's the landlords' capital which makes development possible in the first place. Developers are just providing a service. Well, that's not entirely fair, as they're also assuming a bit of risk on themselves - if they miscalculate the investment, or conditions change negatively before they sell, they'll end up holding the bag. But, majority of risk is on the landlords' side, as they hold the property for decades.