It's hard to keep friends when everyone is working 40-50 hours a week, have to manage a home, have to keep themselves fit, have to potentially keep a partner happy and then your own family, and so on...
Friendships in adulthood is simply not compatible with 21th century way of living. There is no incentive, no time, no energy and no real reason to maintain friendships. And with age, being alone isn't so bad.