They all have functional mobile sites though, and those can be wrapped into an app-like experience on the client side easily enough.

I basically did that with Facebook back when I still used it. The official app was so outrageously shitty, so I used third-party apps that wrapped the mobile site.

It wouldn’t be great but it’d be good enough for early adopters (mostly technical users). Enough to make a viable niche perhaps, and grow from there.