I think people don't realize just how unreliable the major messaging networks are. There was a Reddit thread a year or so ago where some folks who worked in the telecom industry said that they shoot for a 98% delivery rate with SMS, i.e. one in every 50 SMS messages will just get lost. I've personally experienced arriving at a friends' house, sending them a text to let them know I'm there, waiting 10 minutes, knocking on the door, and then 20 minutes later, while I'm in the car with them, my own text message finally arrives.