People do lots of stuff sick. That is why we have pandemics. Do airlines refund your ticket because you're feeling a little under the weather? Nope. So you infect the entire plane-load of people. Do bosses or customers like it when you're gone for two weeks because of a cold? Nope. So you go to work sick and a week later, everyone in your office is sick. The incentives are always aligned towards loading yourself up on cold medicine and going about your day. No company is willing to take the financial hit for public health.

If people disappeared into self-isolation whenever they had a sniffle, many diseases would be completely eradicated. They don't, so they aren't. (Yes, I know the flu can jump between humans and birds and back, so isolation wouldn't eradicate the flu. But a lot of other diseases would be completely gone.)