Okay, but what if their carelessness about privacy directly or indirectly affects you? For instacne, if someone in your contacts doesn't prioritize privacy and has granted apps access to their contacts, it could potentially expose your information indirectly.
Gallery permissions too. It doesn't matter if you've sent them something via a private&secure app, if they don't care about gallery/file permissions that could potentially be exposed.
You can send them an address or any other data you want to keep private via private app, they copy it and voila, it is in their unsecured keyboard. + If they don't care which app grants clipboard permissions, ah...
Also, it's a silly example, but let's say someone uses the same username on every network. They become an easy target for OSINT. If they follow a few people, including you (and worst-case scenario, only you), it's very easy for someone to reach you through that person. As someone who has had a few stalkers in life (not just in the digital world but also in real life), every little detail like this matters.
People who don't take care of their privacy but have contact with you expose you in various ways. It's challenging to maintain privacy, no matter how many phones, phone numbers, or spoof accounts you have, if you want to be friends with someone who doesn't prioritize privacy.
I was focusing on the notion of 'Not my business. Their data', because when our data becomes their, it is not solely their data anymore...
Plus, their data becomes a bridge to ours.
A bridge that connects our information makes privacy a shared concern.