It's not that they don't want members, it's that people assume all unions are massive machines capable of turning your life around in a second, where in reality most of them will give you some legal packets so you know your rights, and a pat on the back if you take your employer to court. There isn't a large restaurant union in my city, just a few small ones that are focused on single businesses. We all have to start somewhere, but just up and joining a small union isn't going to help when the owner of the business can just let you go and hire someone else who isn't unionized.