The failures that return 200
A deploy ships with a broken build step and every page renders empty inside a valid response. A database connection fails and the template catches the error, so the product list is blank and the status code is fine. A third-party script stops loading and the checkout button never appears. An API key expires and the page renders with all its furniture and none of its data. A CDN serves a cached copy from before the last release, indefinitely.
Every one of those is an outage from a customer point of view and none of them registers as one. The server is up, the response is a 200, and the monitoring dashboard is green throughout - which is why the first report so often comes from a customer rather than from the tooling that exists to prevent exactly that.
Watching content is a different question from watching status
Checking whether the page still contains what it should is the check that catches those, because it is testing the thing a visitor cares about rather than the transport underneath it. If the page that should list products is suddenly much shorter, something is wrong regardless of the status code.
It is also the check that notices changes you did not make: a page edited by someone else, terms updated by a supplier, a price changed by a competitor, a certificate page or a policy document altered. Those are not outages at all, and they are frequently the thing you most want to be told about.
Alerting that people still read
The failure mode of monitoring is not missing an outage, it is alerting so often that the alerts stop being read. A single failed check means very little - networks drop packets, a server restarts, a deploy causes two seconds of refusal - and a monitor that shouts about each one trains everybody to ignore it within a fortnight.
Requiring consecutive failures before alerting is the standard defence and it is not optional. The moment an alert channel becomes noise, it has negative value: the real outage arrives in the same tone as the two hundred false ones before it, and gets the same amount of attention.