That isn't true in for every country.

In Belgium, you technically don't get sick days, i.e. there is no specified amount of sick days you can take. A doctor's note is your sick leave. If you have a note, you're good to go. No matter what the note says.

The worst thing your employer can do when they disagree is to send a second doctor to confirm the diagnosis. And that only really happens in companies that have high employee absenteism (e.g. McDonald's, from experience) or in special cases for an employee who is sick way too often.

The punishment for making a sick employee come in are severe, so companies are deincentivized to do so. There has been a lawsuit because an employer called their sick employee for information (and was subsequently punished at work for not being available).
I've never encountered a company that doesn't listen to doctor's notes. They simply can't get away with it. At worst, you'll work at a company that's less eager to allow you to go home if you feel unwell, and you even rarely hear about that (usually more of a manager fuckup than a company policy)

Also, your wages are covered when you're sick (on a doctor's note). The first 30 days, your employer pays your wages (they recover them from a federal fund down the line). After 30 days, your health insurance (practically free in Belgium) takes over. After another year of that, you will be put on disability and get less than your usual wages, but you're still earning at least minimum wage (guaranteed).

The American approach to sickness (and employee benefits in general, also healthcare) is abhorrent to me. I don't see how anyone stands to benefit from such a system in the long run. It's creates conflict where no conflict should be. Of all the things humans divide themselves by, you'd think they'd all agree that we should be united against medical problems that affect all of us. And yet...