It's easy to make statements, suggestions, and opinions, but it's really hard to actually do things in practice.
We all want more people here. The question is "How?", and simply saying "Make good defaults" is easier said than done (what are good defaults?) and "good defaults" is too simple of an answer to a very complex issue.
The conversation has been stuck here for over a year already as very few people are actually willing to develop and test these solutions which takes a lot of resources too.
The irony of the situation is that a lot of these solutions that have been suggested also need a central authority. For example, people have suggested creating a central hub where everyone can sign up to. Another example is creating a little quiz which begs the question "Who decides which instances goes there?".
The best effort I've seen in trying to implement a solution are Reddit mirrors which aren't well-received because nobody likes talking to bots.
As a personal anectode on "lowering the barrier of entry":
Since I started this instance (ani.social), my goal in mind was to make it very easy for users to sign up. There's no manual approval here (except when it's being flooded with trolls). Only email verification is needed -- just like every social media platform.
But sometimes when new users sign up, they comment or post on communities in instances that defederated from us.
Now I have to think about how I'm supposed to explain defederation to people, and the moment I do try to explain, that's a big turn off for many.
Trying to hide federation is impossible. The Fediverse demands a new kind of usership that understands how the network works.
Again, I'm not saying Lemmy can't improve (it can in a lot of ways), but it all boils down to developer resources and who's willing enough to test these solutions to see what works and what doesn't. On the other hand, it takes minimal effort to say "It needs to be better".