Maybe people turn on ban lines because they're tired of users who want to impose their own idea of "fun" and "harmless pranks" on them. You don't get to dictate how other people use SL. Either they'll want to interact with you or they won't, and just like in Real Life don't be an annoying asshole and "no means no". So when a friend and I are standing on their Mainland parcel discussing the build they want to do and some jackass kept walking around us and bumping into us, then yes, that asshole first of all gets yelled at in local chat and then gets blocked/banned from the parcel. Because they're an annoying jackass who does not get to impose their idea of "fun" on us or dictate how we should use SL. We have things we want to do in SL, we are not in SL to amuse narcissistic creeps who thinks the entire (virtual) world revolves around them.
The problem I'm trying to outline is that you can only find social interaction in specific places in SL while everywhere else serves as a cozy bubble for anti-social people who don't wish to have social interaction in a game made for social interaciton (among other things).
How is it a problem? Maybe people like having their own spaces to interact privately with their friends or to work on builds, and then go to the bars/clubs when they feel like socializing and meeting new people? Like...you know...the real world? *gasp!* I have friends from around the world, so SL is a way for us to meet up "face to face" and socialize. When I'm sitting on my couch in the real world having a conversation with a friend, should I have to put up with some jerk who barges into my house and demands attention? No? Well I shouldn't have to put up with jerks in my virtual house either. Go to a club if you want to socialize with strangers and stay out of people's private parcels/houses unless you're invited. And maybe if you stop with the "harmless pranks" and use some common courtesy, people would be willing to interact with you in SL.
And what's wrong with cozy bubbles anyway? I don't recall anything in the SL TOS that said I am required to socialize with anyone. If I want to pay for a parcel and spend the entirety of my time in SL building things without talking to anyone else, I can do that. I am not required to make SL a safe space for entitled extroverts.
Also, if you're unable to find anything to do in SL, you're not putting any effort into looking. There's announcements of events whenever you open the viewer (SL Viewer and Firestorm). You can go to the destinations tab to find themed sims and events. You can search for things you're interested in to find groups/sims -- Steampunk alone has at least four large communities. It's not the responsibility of other random SL participants to entertain you, put some effort into finding areas/groups that fit what you want to do in SL, and if you can't find anything that fits you, make it yourself. Form a group and/or rent land to create what you like. SL is supposed to be user-created, after all.