> Unfortunately, the vast majority of the Belgian Coast is now full of such buildings.

Who are the developers and the owners of these buildings?
Most of them are empty from September till May (or even June), aren't they?

This type of building was massively built in the 1970s by all kinds of building promoters, who eventually sold the apartments separately.
They were promoted as 2nd residence and most owners did 3 things with them: out of season staying there during the weekends, loan the apartment to their children during the school holidays and try to rent it out during the summer months. With the revenues of the rents of July and August, they could already get a lot out of the costs.
That's how it used to be. The traffic to the coast has become so busy that it is no longer a pleasure to drive there every weekend. The apartments are now outdated and the costs of the communal parts (roof, elevators, outside walls...) cost a lot of money as well. The national and local authorities are also going to massively impose taxes on 2nd stays. In addition, there is also the fact that fewer people are willing to rent an apartment by the sea for a longer period of time (e.g. 2 weeks). These kind of apartments are now massively for sale at rock-bottom prices, but the costs to the common parts weigh so heavily that many are empty for a long time.