Yes it's very easy to just let things sit at the "acquaintance" level. You know people at the gym, or at work, or at whatever recurring thing that brings you together. But to extend that to friendship, you need to invite them to do something outside of that. Get lunch, come over for dinner, game night, movie, whatever you're into. And then they need to reciprocate at least somewhat evenly. If this doesn't happen it's not really a friendship. And (in my experience) it's very rare for adults to progress beyond the "acquaintance" stage.