Because Amsterdam is and was an expensive city. As you can see, only the older houses have these hooks. It all started about 300 years ago.
When these houses were built, the tax was based on the width of the houses. As humanity has always done, Amsterdammers decided to play it smart and save on taxes. So they kept the houses as narrow as possible. Even if that meant doing away with the wide staircase.
The problem now was how they could move the furniture up that extremely narrow staircase to the upper floors. The answer was external hooks and larger windows. The idea was to hang the furniture from the outside and pull it into the house through those windows. And it worked. It still works.
Take another look at the photo above. Do you see that middle house leaning forward? Yes, not a structural defect. This was done on purpose. The slope helps prevent the furniture from bumping into the walls or windows of the house. As I said, it still works.