Partially because these large systems have some kind of ongoing issue at any given time, so it's challenging to provide a meaningful live status that isn't a bit misleading and could cause misdirected panic.
Partially because you don't want to give potential attackers (eg ddos) any insight if/how their methods are affecting your systems.
Partially because there are SLAs and reputation at risk, and you don't want to admit to any more downtime than you absolutely have to.