I've been to Pompeii. I was amazed how far away Vesuvius is from the city it destroyed. Similarly, I've seen photos of Mount Saint Helens from the lodge on Spirit Lake which was destroyed when that mountain erupted, and that volcano also looks too far away to be dangerous. The difference is that Mount Saint Helens gave months of warning and Mount Vesuvius allowed only a few days from the first rumblings to utter catastrophe. And apparently a good percentage of the population of Pompeii did flee, or try to; they say the area all around is full of skeletons, anywhere they dig.
But Mount Vesuvius had been quiet for centuries, plus knowledge of volcanos was sparse in ancient times; the soil was wonderfully fertile, the bay was beautiful and the mountain view spectacular, who wouldn't want to live there? Look at modern day Naples, over 3 million people live in and around the city. How many could evacuate safely, with less than a week's warning? And they know they're living next to an active volcano. Vesuvius ceased it's last modern eruptive cycle in 1944, 70 years ago. Few living people remember the smoke plume on the summit of the mountain. So they live their lives, and accept the risk; because there's nowhere on Earth that's totally safe, and the land is still fertile, the bay is still beautiful and the view is still spectacular.
Good times and bad times; that's life.