If I installed a different app for every friend I had, I'd have a homescreen full just of chat apps. What's worse, those niche privacy friendly apps go under or out of favor often.

You might be able to convince some of your friends to install an app just for you once, but by the time you're telling them "this one now sucks, I'm on other app now" for the second time, they'll just stop chatting with you, and if you ask them repeatedly, likely shun you even IRL because most people want to live their lives, not chase chat apps for their friends' weird interests.

And even if they do that, they'll have one app that they use every day, and one that sits in the bottom of their app drawer. Guess who gets invited to do something on the weekend, the person who shows up on their main contact list, or the person that would show up if they dug out that dusty app? And guess what the phone is gonna do with that app once it hasn't been opened for a week... it's going to deprioritize it so it won't even work properly, while their main daily-opened app always gets push notifications immediately.

You don't have to like it. You can pretend it's not happening. But it will happen.