Sometimes it is, say here: https://fedidb.org/current-events/threadiverse
But you might as well say why isn't Mastodon included? Kbin is a fundamentally different platform and architecture, even if the two are largely compatible (as is Mastodon, to a slightly lesser extent with Lemmy).
It is perfectly reasonable to look at the growth of Lemmy, think of it like quoting use of one flavour of Linux - Linux uptake overall is also interesting, but not the same.