A colleague of mine came back from Milan just a few days ago and he was just sitting there in the office, as nothing was/is happening. Only after some us got pissed, HR anonymously sent him an email to ask him to work from home for the next weeks. Same for anyone having a cold or any symptom, or flying back from one of the countries currently facing the coronavirus
People need to be told what to do, otherwise they don't think they are harming others. This behavior is even more common among young people without family. The moment you have a child, you start to look at all these potential threats and react accordingly.