Traditionally they don't need to care about showing you good content to keep you interested, you're generally logging in anyway.

The proof is in the pudding: if you want a curated feed go to Tumblr or Mastodon. These spaces exist yet they're significantly less mainstream.

Why would we ignore the very mainstream Instagram as an example?

> you're generally logging in anyway.

When an app is established, sure. But providing new users a shitty experience in order to prioritise revenue is how new social networks fail.