That's because liftoff aggregates posts from multiple instances on its own, rather than relying on instances "federating" with each other. The benefit of this is that it gives you control of what instances you see content from, rather than the instance owners deciding for you.
Personally, I think that makes way more sense than making instances "federate" with each other to share content. The only interoperability instances really need is the ability to post to communities that are hosted on other instances. We don't need instances to be sending the entirety of all their content to each other; clients can aggregate that on their own.