I'm sure there are lots of people who aren't friends with their coworkers, and it sounds like the coworker you mention in particular falls into that category if she actively tries to avoid seeing coworkers outside of work.
It is definitely possible to be genuine friends with coworkers though! Even though I moved away three years ago, I'm still friends with someone I used to work with, and am in fact going to her wedding this summer.
In terms of not hanging out with them outside of work, sometimes that might be because you already see an awful lot of them at work. I see and talk to one of my friends every weekday at work and also text when we're not at work. That's significantly more than I see and talk to my non-coworker friends, even if we almost never socialise outside of work.
I hope that answers your questions?