They don't do it more because they know most vacant property is owned by regular folks.
It's just a supply demand problem here. People in Spain just refuses to believe it because there are some narratives circulating, specially in the left, about how the housing bubble ruined spain so constructing is speculation and bad capitalism yadda yadda.
They don't want to hear about public housing neither because "it's expensive" while they advocate for more public spending in other useless stuff. They want price control and such.
The whole situation is just so stupid.
Do you read spanish? I may find something for you, but probably not soon.
I don't think there's a supply problem that would be fixed with new buildings. Last governmental statistics shown that there were at least 3.4 million empty homes [3]. Statistics coming from the private sector (real estate agencies, and a private university) this year, they calculated a 10% of vacant homes nationally [4]. That's a lot.
I think there are no good statistics on the quantity of homes that are owned by "regular folks". Also, what's a "regular folk"? Is my landlord, who owns more than 50 apartments in top notch locations, a "regular folk"? (Not counting on the illegal things he's done)
That's why you can't only focus on big companies, like Blackstone (they own 20 billion € in real state just in Spain [5]). A study that has been making the rounds this year on all the economic press [6](pages 22 and 25) has been used, wrongly, to show that "big landlords" only (hah, only) own 4% of all the rental homes in Spain. But if you check the list, it only shows the top 40 companies, being the number 40 in the list owner of 121 homes. Also, I don't see any private owners.
Do you stop being a "big landlord" if you "only" own less than 121 homes? I can't see my landlord in the list.
Truth is, there are empty houses. The rental pricing rise doesn't correlate to salary rises, specially in big cities like Madrid or Barcelona. In less than 5 years (that's the time I stayed in my last apartment) I've seen prices rise 30 to 40% in Madrid. Before COVID AirBnB didn't help either [7], and 30% AirBnB owners were proprietors of at least 5 houses [8], so, pure speculation. And although people (specially those who can WFH) have been leaving Madrid after COVID, rental prices haven't lowered that much[9].
Sorry for the long rant and my terrible English.
1: https://atc.gencat.cat/es/tributs/impost-habitatges-buits/
2: https://www.eleconomista.es/vivienda/noticias/11325256/07/21...
3: https://www.europapress.es/economia/noticia-cuantas-vivienda...
4: https://cincodias.elpais.com/cincodias/2021/02/02/economia/1...
5: https://cincodias.elpais.com/cincodias/2018/09/17/companias/...
6: https://atlas-reanalytics.com/wp-content/uploads/Estado-y-te...
7: https://www.eldiario.es/economia/concentracion-airbnb-manzan...
8: https://www.eldiario.es/economia/airbnb-espana-anfitriones-g...
9: https://www.20minutos.es/noticia/4639918/0/el-precio-del-alq...
It's just bonkers, but people get angry when you point about the stupidity of such decisions. I even got a few downvotes in HN.
Some of these folks own two, three, and four homes that are sitting empty.