I've noticed the same as well. My main hypothesis is that parties are just less interesting to all concerned because they're not that novel, and often just retreading old ground.
When you're younger, parties are full of new experiences, lives are often changing rapidly (e.g. jobs/relationships/homes), so parties feel like a worthwhile way to spend time.
As people get older and get more settled, then parties often feel similar to rewatching the same movie over and over (sure it's a great movie, but you'll never recreate the experience of seeing it for the first time). Often less is happening in people's lives too ("How's the job?" "Good, same. How's the house?" "Good, got a new sofa. How are the kids?" "Good, difficult as usual.").
Free time also seems to be more of a premium for me and my friends (as work responsibilities grow, kids take time, need lifts, etc.), so events like parties need to be worthwhile enough to give up other things for.
One way that I approach it is to think of what it is I'm be trying to get out of a party. Meeting new people? There are other approaches for that. Keeping in touch with friends? Smaller gatherings or individual catchups work better.
I.e. rather than trying to recreate my idea of what a party should be, I try to break it into individual apsects that I'm looking for, then try to achieve those using simpler/smaller alternatives, instead of looking to a single party event to provide them all.