Because people don’t want to think they are vulnerable. They hear that the mortality rate is 5%, and of that number, 80% or higher are seniors (in other words, a 1% chance for general population), and believe they’re safe. After all, a regular flu kills x number a year, and we don’t stop living for that.
Furthermore, many people believe that religion automatically keeps them safe. “G-d would never punish people during a minyan. Coronavirus only affects bad people. The bochurim are learning Hashem’s word. He will protect them”.
Otherwise, they recognize that being locked away alone is not living, and would rather enjoy life regardless of the risks.