In some areas even national park and bird sanctuaries have been illegally constructed upon and then the government was bribed.
They might class perpetually vacant properties as abandoned and therefore returned to the common. When New world land was doled out in square mile allotments (The US, Australia..), improving the land was a prerequisite to your claim. This is often thought of historically as a deal: 'you improve this land and we'll give it to you.' But the justifications at the time was more philosophical: you improve this land and you own it, by definition.'
If they do, though, it's high time to check if these additional clauses are honored.
This was based in ideas trying to explain what property is, why one person was rights to it and another doesn't. A wild forest is not property. A farm is. Hence labour, habitation, productive use etc. make something property. It's the kind of philosophy that gets very theological, with precise definitions and arguments between theorists that seem very arcane an pointless in retrospect.
I'm not promoting the theory. It just seemed relevant in the context of a liberal/libertarian quip. These ideas are foundational to ideas of private property, later economic freedom etc. It's part of the history of ideas related to right leaning politics today.
In practice, I'm not sue if this is workable. But if it is, I don't think an abstract sanctity of private property is necessarily a barrier. That unused property makes fro a weaker claim is justifiable by common sensibilities, right or left wing philosophy.
Property rights in most countries are mitigated by a variety of competing communal rights such as eminent domain, zoning, and environmental laws. I think squatting rights could potentially protect communal rights in a similar way.
I suspect that squatting rights only affect obviously neglected property. I suppose the empty houses and apartments are reasonably guarded and are not as neglected as a closed and half-dismantled building. Also, squatting does damage the property, hopefully in minor ways. Imagine that your empty backpack was loaded with pizza slices: totally benign, but you'll have to clean the backpack thoroughly.
As long as governments continue to rob their citizens via taxes, people will do their best to not be robbed, resulting in "black money" (which is actually "white money" since none of it went to fund wars and murderous statist schemes, as opposed to the "red money" which is tax paid and so did help fund wars and statist aggression against peaceful citizens).
This "white money" will then find a place to be invested where it receives some ROI, unlike the banks and bonds and stocks that have been completely destroyed by government meddling in the economy. That is real estate or precious metals and that's where the "white money" will go.
The entire "problem" (if it is one at all) is created by statist violation of property rights through taxation, in the first place. Further statist property rights violations will only distort things even further. Let's say you introduce squatting "rights". Well now you know what happens? Nobody's buying real estate in your country any more. All the "white money" flows to alternate investments and real estate in other countries that don't have squatting "rights".
It's basic human action. People don't want to be enslaved and robbed from. And they will do everything in their power to avoid it.
Why not? Argument "scope creep" :)
As others pointed out, squatting can be used to keep private property owners (especially of such basic necessities like housing), from hording/speculating.
So why? BBC basically answers with the following: because some people earn more money this way. Tell me something new BBC.
Let me posit another reason: because squatting these empty houses is not allowed.
If squatting these long-term empty places was allowed they would be:
1. sold/rented at a more fair market price,
2. squatted (in which case they are also not empty anymore).