If house prices appreciate, why do developers sell them? In many parts of the world, people buy a house expecting its value to increase over time.

But if is widely believed that the building's value will increase, what incentive is there for the developer to sell the property?  Why not hold onto the property, and sell it later at a higher price?
They are different businesses.

Developers make more money developing than landlording. Some do both. But they are different businesses entirely. Also, there is a limited supply of capital and carrying that capital has a cost.

For example. Let's say Bob the Builder borrows $10M to develop a project. The bank lends on the development. Not the completed project. That requires a different type of loan structure, depending on the use: residential vs. commercial, owner occupied residential vs. residential rental, etc.

After the project is completed, Bob will usually look to sell. If he holds the property as a rental or a commercial, he then switches hats and becomes a landlord. And the asset will require refinancing, as above described. Some builders choose this route. But most do not. As their skills, expertise, equipment, personnel, interests, etc. are all used best and most profitably building and developing.