My grandmother died some years ago, my family wanted to sell the house as they didn’t need it. It took 3 years to sell it because it’s in a location with no demand plus 6-12 months to arrange the inheritance legal details. You would say, the price was too high then! It was sold for €30K, a garden farmhouse with 6 bedrooms, garden and garage.
It’s inevitable that some houses are out of the market for some time due to many different reasons.
Zero unoccupied housing is unachievable anywhere but in a dilapidated slum, because apartments must get remodeled from time to time. If you remodel an apartment every 15 years and it takes three months (a rather optimistic timeline), you already have 1,6 per cent of the housing stock out of the market inevitably, because 3 months is 1/60th of 15 years. If anything, only 2 per cent of the Berlin apartments being out of the market indicates that they already don't get enough maintenance!
Then you have pauses between tenants. If an average tenant stays for 8 years and it takes about a month to fix the most basic things once he moves out, and find new people, then you have 1 per cent of the housing stock momentarily unoccupied just due to this. Again, Berlin having only 2 per cent of the stock out of the market indicates that the changing of tenants is already overoptimized, with not enough time to vet new ones and fix smaller things: one moves in on the heels of another.
Then you have a certain small percentage of apartments being part of litigations, either commercial or inheritance-related. Inevitable. Companies split, people die, and not everyone has their papers in Ordnung.
You seem to be claiming that all those mechanisms can be compressed to zero, and that all the apartments that are unoccupied due to them can be put on the market by fiat. How? Will you ban apartment remodeling? That's the way to produce - fully occupied - ghettoes.