Because it was forbidden by law. You were forced to be put in an institution or directly went to prison. Check Section 249 of the DDR Criminal Code.
And by the way, the DDR suffered from chronic housing shortage. People were cramped in barracks with not bathroom and no heating. Technically housed but in actually povert level of housing. It was an absolute disaster.
Seems like a very good law, don't you think? And this reminded me of how come one of my favorite childhood books from when I was 3 or 4 here in Romania was about DDR constructors building apartment flats, loved that book and loved the spirit behind it.
> People were cramped in barracks with not bathroom and no heating
There were? Back in the '70s and '80s? You sure? I don't remember it that way here in Romania back in the '80s, and I know that the DDR had a much better economic condition than us, Socialist Republic of Romania.