I know it's always been a bit of a financial asset, but the past 40 years have really seen this accelerate to pretty crazy levels. The issue, when you boil it down to the basics is: wealthy folks, development companies, overseas oligarchs, hedge funds, etc. owning many properties where they try to extract as much value as possible. This means that rents go up and short-term-rentals become more viable. This also means that young middle-class families can't afford to purchase or to live there, so it pushes out locals (this is especially bad in Lisbon).
The solution seems to be government telling you what you can or you can't do with your private property, which does not sit well with me, but it's becoming more and more clear that this can't go on forever.