There's poor memory, depression, ADHD, low-level addiction, social anxiety, poor memory and a great number of other things that can keep one from contacting friends.
There are many people in this thread who clearly do care and have trouble doing/remembering, so I'm not sure why you'd choose to respond this way to someone you're more likely to hurt.
Yeah, it's seems like poor empathy or a lack of imagination or life experience to not be able to conceive how people can forget things they care about. We'd all do well to remember that any given thing that's easy for us might be difficult for someone else, and not necessarily through some personal failing on their part.
There are many people in this thread who clearly do care and have trouble doing/remembering, so I'm not sure why you'd choose to respond this way to someone you're more likely to hurt.