The most common failure on a live site is not an outage. It is something that returns a perfectly healthy response code while being completely broken: a stylesheet that stopped loading, a form that submits into nothing, a page that renders but no longer contains the thing it is supposed to sell.
Every one of those returns 200. Uptime monitoring reports all clear. The business finds out when someone mentions it in passing weeks later.
So checks here look at the body of the response, not the status code. Does the form actually deliver. Did the CSS actually load. Is the page still the page. That is a more annoying way to monitor a site and it is the only kind that catches the failures that cost money.