That's sometimes how it goes. I'm in my late 30s and been through many phases in life. Different careers, different cities, etc. It's very difficult to keep friendships together when the circumstances that kept you in the same place change. After university, people go in dozens of different directions and disperse around the country. If you work in some job for many years, you'll find the same thing to be true when you leave.

There's only probably a handful of good, close friends that will last for more than a few years as good, close friends. That's okay. You'll make new ones in the new endeavors you take up. Do what you can to hang on to the ones that stick around.