This really doesn't ring true outside of school/university years. Back then, you are constantly meeting new people, and seeing them regularly in class and on campus. People that age don't have a "max occupancy" number when it comes to making friends. As an adult in the working world, the opposite is true.
Outside of that uni environment, a new person is going to have to knock your socks off with their personality, for you to even consider meeting up with them again. Most people cannot make a first impression that good.
Outside of school/uni, people are busy with daily life, and far less likely to chat with strangers. Friendship groups begin to quickly narrow around this time as well, as people get married, go to professional schooling of some kind, and start hanging out mostly with people that are very similar to them.