In my experience it's because the monitoring system is usually controlled by another team. So you they don't know what they should be testing, and the developer

In my experience it's because the monitoring system is usually controlled by another team. So you they don't know what they should be testing, and the developers who know how to do it aren't easily able to set it up as part of deployment. Add issues like network-visibility, and you wind up talking about a cross-team, cross-org effort to stick an HTTP poller and get the traffic back to some server - and so going into production without it winds up being the easier path (because it'll work fine - provided nothing goes wrong).