A lot of friendships aren't that deep. I've had work colleagues I really liked and even socialized with outside the workplace, and yet, if they left the company or retired... faded away. If it takes real effort to keep up contact, you get a lot more choosy.

COVID also hit pretty bad. Speaking from personal experience, several friends that we saw once or twice a year at informally recurring BBQ/brunch/etc. kind of occasions have faded away as that series was interrupted and never restarted.

And finally... you learn who you are as you age. Friends who seemed cool, who seemed to have the answers... may not be so great from a mature perspective.