i'm one of those expats myself. The reason is
A. its' not financially possible to go to bars every night. (Unless your a college student and your mom and dad pay for everything, in which case good for you!! ) I'd say it's possible to go out once a week max for myself.
B. After 3 years, I've done everything you've mentioned like 8 times, been to tigre probably 40 times (used to live in zona norte and I have friends there) I would love to find something off of the path though and I"m always looking for new things to do, but you have to understand not everyone likes to walk around and look at the river in puerto madero with other expats, gossiping about other expats
C. Expats are great, but I try hanging out with Argentines or the expats/immigrants from other latin american countries because Buenos Aires has un poco de todo from all latin american communities. Think of it this way, let's say you are interested in watch repair and you go to a watch repair seminar in Switzerland. Would you hang out at night with a watch repairmen from your home town or a watch repairmen from the other side of the world? I'm sure everyone's answer would vary and that's the point
Thats not to say I never hang out with expats from my country, I do, and they are awesome people, but if I do it all the time I find myself doing the same stuff, talking about the same stuff ,seeing the same stuff in a place that screams diversion. It gets a little depressing.
So, with all this cynicism, you may be asking why I'm here. That's a long and complicated story but why I choose to stay is because I like the life here, I like walking down my neighborhood and knowing who everyone is and them knowing me, speaking spanish to everyone, calling the peruvian fruit vendor a webon, having the argentine kiosk guy tell me he's glad we tied england in the world cup, sneaking popcorn into abasto movie theatre because they only sell dulce popcorn. it's the little things. The big things have gotten old and frankly bore the piss out of me when I realize hey, I've been to Plaza Francia 50 times and it's the same stuff (that's why you usually only see tourists there with a few expats who are proud they know their way around)
So, I hope that answered your question. If your looking for expats to hang out with, try facebook or this forum as there are billions of events that expats go to, but also realize there is another type of expat, probably was a goth in high school, but LOVES being out of the same loop everyone is trying to get into.