I get that Mastodon has most of the people on the fediverse, but that doesn't mean it's on Mastodon to feed activity to other platforms. Even if it wanted to, I don't think it could.
Even if we ignore the issues, I just don't think other ActivityPub platforms would benefit much from this. People are going to Mastodon for microblogs, not all the other stuff. Even if you give people the option to view PeerTube videos from Mastodon, if that's not what they're here for, then most of them won't bother. We see this on other platforms. Even on Kbin, which has great support for the microblog format with a unique tab and all, a ton of the people who came here for a Reddit alternative just don't bother with the Twitter stuff.
And again, that's ignoring all the downsides of implementing this stuff. Add all of this non-microblog stuff, and more people who are searching for a Twitter alternative will disregard Mastodon as too cluttered and confusing. People already do that because of needing to pick an instance, and yet more do because they can't immediately grasp federation. Additionally, those who do interact with, say, a Lemmy comment through Mastodon won't be aware of the context behind it being in a Lemmy thread that's part of a community and which has different functionality (e.g., downvotes & a title). You could have a badge to indicate that it's from Lemmy, but then you worsen the problem of people getting to Mastodon and going, "Wait, wtf is this? Why is this platform so confusing?"
And let's say Lemmy/Kbin threads did get some great surge in activity from this. You'd end up with a significant portion of activity on these platform being people using a Twitter alternative, which I don't think people looking for a Reddit alternative (or even people already on these platforms) would find desirable.
Since you brought it up, I think Lemmy's handling of microblogs is a good example of what I mean, as I don't think Lemmy should be handling microblogs at all. Lemmy has to assign a title to something that doesn't have one and downvotes to something that doesn't accept them. The result is that several Kbin magazines, when viewed from Lemmy, are dotted with posts that obviously aren't meant to be there and which would clutter the thread to death if microblog usage on Kbin increased. And for what? When I make a microblog post, I don't intend for it to reach people on Lemmy, as that format is not what most people on Lemmy are interested in. Likewise, most people on Lemmy aren't trying to reach people on the Twitter alternative and might prefer not to. Lemmy supporting microblogs doesn't really do much, and the same would go for Mastodon even given its large user count. People just won't be interested.