What is up with the three day rule?

For real. You know what I’m talking about. Why do guys feel like they have to wait 3 days to call a girl once they get her #? By the way, of course I know this isn’t always the case. When I met my ex he said “I’m calling you tomorrow” and he did. My roommate just went on a date, was convinced the guy didn’t like her, and then called her today—- 3 days later. What’s up?