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.)

Usually symptoms linger long after you're contagious

I don't think this is right. Getting sick significantly more or less can be a drawback at the individual level. Your hypothetical self-isolating people would be more sickly.

Most of the work humanity does against a given pathogen is in getting sick. We've recently automated that. But our immune systems are still out there fighting the good fight every cold/flu season.

The ideal season for an individual is one where you get exposed but don't develop symptoms, not one where you never get exposed. Not getting exposed is like missing a ski trip-- a terrible outcome to be avoided at all costs.

At that point it was a cold.

My wife occasionally works in an emergency room, so the advice she gave me is the same advice that anyone else with the same symptoms would have from a doctor following the CDC's instructions.

She just thought it was a seasonal virus.

Edit: It wasn't until mid March that we (MA residents) suspected it was in the wild. Basically, there was a reported case that came through Logan airport in February, and then when it came out that the incubation period was 2 weeks and many cases just resembled a bad cold, everyone started to suspect that there were a lot of asymptomatic cases in the state that we didn't know about.

The CDC didn't see the first case of community transmission until Feb 28, a week after I got sick.