Probably because it changed ownership they didn't confiscated it, when someone bought a house they did paperwork which took some time, rinse and repeat. It actually works till someone will care.The house real owner didn't pay tax for how long the author didn't specify. Why didn't the government confiscate the house long ago? I didn't know the tax rate on that world but total 150 million yen tax plus interest, maybe it hasn't paid for decades. Big houses are pain to clean.
Dragons are endangered animals, please don't hunt it. The mc should buy another house.