Because Threads is Meta's attempt at bullshitting Mastodon users in welcoming a wolf among the herd. Search for "Fedipact": Meta is de facto cut off from many Mastodon instances.
Except the largest Mastodon instance, mastodon.social, does federate with Threads. I'm not even sure if the list you provided even covers most of the top instances either.
It really feels like an "eating your cake and having it too" kinda situation: you get the engagement and interaction with millions of Threads users but you don't have to count them in your decentralization metrics.
