I think there are a few culprits here.

• Not everything wants to be an everything app. While everything in the fediverse uses ActivityPub, that doesn't mean everything has to aim to be interoperable. I wrote a lengthy rant about this here, but essentially, it's important to have things with a more specific, restricted purpose if we want the fediverse to be accessible. If someone just wants a thread aggreegator (i.e., just Reddit's style of media), they shouldn't be forced to grapple with microblogging features more fit for a Twitter-like. There are some platforms that aim to combine different media types—Kbin/Mbin has both thread aggregation and microblogging, and I've heard that Friendica tries to work well with everything. Even so, if someone wants federated Reddit, they should be able to have federated Reddit, and Lemmy aims to provide that. The same way that Pixelfed (an image-sharing platform like Instagram) doesn't need to incorporate Reddit-style threads or Twitter-style microblogs, Lemmy doesn't have to do it all.

• Federation is still in the works. Something to keep in mind is that most of these platforms are early in development and still working out a lot of bugs. Kbin (the platform I use) is an obvious example due to its currently incredibly spotty microblog federation (tho I've heard that Mbin has implemented fixes to fare better in this regard). We have to be patient while all the kinks are worked out. As much as we all wish it didn't, software development takes time—a lot of it.

• Admins can sometimes be a bit trigger-happy with defederation. I don't think the fediverse has quite grasped that defederation is essentially the nuclear bomb of instance moderation tools, cutting off interaction with all users of an instance. While there are times where this is justified (even preemptively, such with Threads imo), there are times where the nuke has been threatened over a quarrel between admins or disagreements about other defederations. Hopefully, this will cool down as the fediverse matures, but we'll have to see how that pans out (especially with Threads federation growing ever nearer).