- Technology out of the ass
- Public transportation being limited to non-existent in plenty of locations
- Malls are not what they used to be
- You can purchase items with a few keystrokes and smartphone taps
- Everything is expensive
- Jobs are limited
- Too many job hours
- Student debt
I used to be all about going out and so forth, but once you hit a certain age and married...shyt is like "seen it all"

. I do lone for the days of just chilling in Center City and just people watch...but yeah

.
Pretty much all this
At one point you had to go out to get the local buzz, chat with people and find out whats new. Not anymore, you can get all the local news and then some from the bed or yard. You want something, you can have it pre order and shipped on release date. Traffic is bad in almost every decent size city and public transportation is trash in most cities outside of major ones.
Like you said going to the mall is more of a fashion show than ever now, it use to be a combination of a place to shop, chill, see whats new and "floss" a bit. Now its strictly to show off, and pick up what you need that you can't find or get online.
Add in the fact that millennials are old enough to be in the tech loop and just old enough to not really have the time nor energy to go out its the perfect storm.
If you worked 60 hours a week have a day or two off the last thing you usually trying to do is spend that day out drinking, or shopping.
Those that do by the time they hit 31 you can tell by the look in the face that they haven't spent enough time in bed and way to much time chasing a good time.
Some people are just social, I understand that as well. But they're is a difference between being social and looking for life out in a club, bar, restaurant or wherever every weekend.