People definitely want to live by themselves even if they cannot afford to, and other things also factor into it.
(Disclaimer, I'm American, so can't speak directly about the Portuguese experience.) But as an example, at this point I don't remember the last time I saw a bunk bed in a non-dorm room setting. This used to be extremely common in low-income housing in particular [1] but as an example I have never, ever seen anyone share a bunk bed with a stranger.
One other effect is location, location, location. In Lisbon in particular, like most European cities there is generally a height limit imposed on most of the city (higher than most places in the US, but not so high that tall buildings are allowed). There is only ten buildings above 80 meters in all of Portugal. If your building is so short, there is only going to be so many ways to subdivide it before you can't split it more to accommodate people.
(Disclaimer, I'm American, so can't speak directly about the Portuguese experience.) But as an example, at this point I don't remember the last time I saw a bunk bed in a non-dorm room setting. This used to be extremely common in low-income housing in particular [1] but as an example I have never, ever seen anyone share a bunk bed with a stranger.
One other effect is location, location, location. In Lisbon in particular, like most European cities there is generally a height limit imposed on most of the city (higher than most places in the US, but not so high that tall buildings are allowed). There is only ten buildings above 80 meters in all of Portugal. If your building is so short, there is only going to be so many ways to subdivide it before you can't split it more to accommodate people.
---
[1] - a historical photo of an NYC tenement: https://media.gettyimages.com/id/640482741/photo/bunkrooms-i...