I think the reasons are:
1. In most places, the absolute numbers are still very low. If you look at it like "X in a million", X is still low, so it feels like there is no need to panic.
2. Continuing the above, if you compare X to something else, like "getting hit by a car" or something similar, it feels rational to not worry about Corona too much, since you're also not worrying about other things (you're used to the risks).
3. People like Trump are comparing it to the flu.
4. For most healthy, non-smoking people under 50, the risks are relatively low based on the data. Kids seem to be pretty safe from it.
5. Based on official numbers it's contained in China: more people are coming out of the disease than going in, the net number is negative, active cases is declining. [2]
However, 1. + 2. above are only re-assuring if you use the "independent events" mental model, which everyone defaults to. Eg. if you get hit by a car or you get cancer, that doesn't increase my chances. So if the probabilities are low (X in a million), and you and me getting it is independent and the probability is not increasing, then the mental model works, and it's reasonable not to worry. But this mental model is wrong here, because this is viral growth. It's spreading exponentially outside China [1]. So you getting it increases my chances of getting [eventually]. We are not indenependent. It's like the flu in this sense, but it spreads more efficicently and kills more people, so 3. above is also a wrong.
On point 4 above, let's hope this is the case. But, everyone has older/sick relatives..
On point 5 above, it'll be interesting to see how liberal democracies respond, whether they can pull the aggressive measures like curfewing the population that China did. Also, China has a lot of economic resources, not sure how smaller/weaker economies will hold up.
In conclusion, I don't think most people realize that for the next ~3 months (imo for a lot more time), their lifestyles will be drastically different. Until now we were in "1st world yolo" mode, now it'll be: no going to work, no schools, no travel, no socializing, no gym, no events.. A lot of the things we work for, live for, will be put on hold. See China, Italy.
[1] see https://www.worldometers.info/coronavirus/coronavirus-cases/, look for "total cases outside of china", it's a beautiful exponential, the kind that startuppers would kill for... "viral growth"
[2] see https://www.worldometers.info/coronavirus/country/china/, look for "active cases in china"