Have you considered that signaling could play a large part into this? A European friend of mine once said, “people in the US try to do everything in high school.” Because, to get into a top undergraduate program, Americans have to signal very hard. Worse, a master’s degree is quickly becoming the new high school diploma, due to signaling to employers.
When kids are spending their lives trying to signal stronger, it’s a lot harder to balance it with friends. It used to be dating as an undergraduate made sense—people would actually get married during or out of college! Now, it makes less sense to date for a year or two and try to maintain a long-distance relationship as you split off into different PhD programs.
I think this needs to be done for >18 year-olds as well. Most research positions require a PhD as a prerequisite, when there are many talented undergraduates who could drop out of college and perform the research after a few weeks’ training.