Kids change it too. It's hard not to make friends when you see the same parents a couple times a week at soccer or baseball or swimming, year-after-year. There's a lot of downtime and you around people that you have something in common with.
With both of our kids we had a great social life with groups of the other parents all through school. There were annual dances, fetes, mums nights out, dads nights out, meeting for a quick beer etc. etc.
Now the kids have left school the majority of that has dropped off, we do keep in good social contact with a couple of people but I really miss how busy and varied the social life used to be.