I thought that by now we wouldn't have to go over this every time.
- This coronavirus spreads asymptomatically, which means people under 50 mingling with each other are deadly threats to people over 50. In particular, to their parents, grandparents, aunts, and neighbors.
- The chance of dying is low for young people, the chance of suffering serious damage to lungs and other internal organs that takes months to recover is a bit higher (some of it may be permanent, but that's yet to be determined).
- The chance of dying for everyone goes up several times if the medical system gets DDoSed by COVID patients.
EDIT:
> It makes little sense to force young people to stay at home and get fired
One of the most important effects of the lockdown being done top-down by governments is that employers can no longer fire people who wanted to protect themselves, their families and their communities from getting sick. Leaving this choice entirely to individuals is in practice equivalent to forcing them to choose between staying safe and having a job.