Because the government won't let you die. Say that person that said "I'm opting out" loses their job and maybe gets a chronic sickness. Now they have no disability, no food stamps, no medical. They're literally dead if they don't have someone they know willing to support them.

Say one family member works and the other is a stay at home spouce and they have 2 kids. Same thing happens, except they're all fucked. Can't use the resources because "you opted out of that stupid shit".

Everyone wants to not pay into it until they need it. I'm not saying it's handled the best, not it keeps people from dying.