White rice is not dangerous per se. Eating it won't kill you. The problem is that people nowadays move much less than they used to do it for millennia, because of changes in the way people get around (cars), entertain themselves (TV/internet), and work (office/knowledge work). Carbs provide high calories which made them popular in the times where any work was mostly physical work and people needed more calories. Nowadays, the modern lifestyle requires less calories therefore meals that served people well for millennia are no longer suitable as they provide too much calories.

There are other factors -- dramaticaly increased affordability and abundance of food, and mass use of taste enhancers in food (such as sugar and salt) that make people overeat, but that's another story.