In the US, snowbirds often live in New England/New York/upper Midwest in the summer and in Florida for the winter. Not many people want to only rent for the 6 suckiest weather months the places are otherwise empty (meaning such tenants can be hard to find, not willing to pay enough to make it worth the hassle).

Say you buy a vacation home and want to be able to use it spontaneously. Then you have to put it on Airbnb for short-term rental, and have a property manager and cleaner. All that costs money. Then you have to pay taxes on that income. On top of that, you have to take on the whole risk yourself that the renters don't trash your house. Yes Airbnb has some program but they also have a great legal team to defend themselves from paying out.

For many people it just isn't worth it.

Why? >>an obvious amount of massive empty properties should, according to theories of supply and demand, lead to low rents

While this might seem true in theory, the reality is obviously different - multiple companies and individual landlords find it economically viable to keep them empty some or most of the time.

There can be many reasons for this. Maintaining a high-value property is not cheap, but maintaining it (and the neighborhood/area) as a high-value property can command disproportionally higher rents, more than enough to offset low daily occupancy rates. E.g., renting it out for 10 days every two months at $10K/night is far more profitable (+25%) vs keeping it fully rented out at $1000/week - and there's less wear-and tear, and those are low rates.

>>can find someone that is able to pay

the point is that if you build and maintain a property of sufficient value (view, amenities, etc.), there are plenty of people willing to pay enough to make it worthwhile, and they have the bank accounts to eliminate risk of non-payment. And it is often less of a hassle than trying to keep a place 100% occupied at low rent.

And, if you are willing to do it as an individual, you can have a far better vacation home by occasionally renting it out than you might otherwise have.

tl;dr: High quality, high rent, low % occupancy by time can be a better business model in the right locations.