Life, travel, location etc have exacerbated the issue but even though our daughter is all grown up a lot of our friends are still middle or early years raising families which is a huge time commitment so they aren't throwing many parties either.
In many ways I lived that lifestyle, hard from 17-32 and even then I kept having to make younger and younger friends to keep a party crowd with me because with maturity comes obligation.
I would rather poke around my yard, hop online and game with friends and family and not have to deal with crowds, drunk people, and etc and I bet a lot of your friends fee the same way.