1. You're going to be working and single/married but still have plenty of time to spend after hours and on weekends with friends. The crux comes from income disparities that might prevent some friends from spending money on activities that others are willing to do.
2. As soon as kids happen, the group dynamics shift. The parents are often times going to be exhausted for a few years (especially if both are working), going out requires hiring a babysitter, inviting people over involves working around the kids or trying to avoid waking them. We host our friends all the time just because it's easier, but it's very different. Overtime this shift in dynamics becomes a big deal.
3. The solution for getting together with friends when you have small kids is other people with kids the same age who can play together. This causes another significant shift in the dynamic as you start to create new friends with parents from your neighborhood, school, church, scouts, etc. If you're in an new area and many people in these groups already know each other, then you're going to be responsible for making the effort to get to know them.