It's not that complicated.
Threads is another instance that brings people to the Fediverse, and people like the idea that they can stay on their instances while still interacting with the world at large. For many people, having everyone on the Fediverse is the goal, and in fact, is a long-term goal of most of the platforms - the "Fediverse" is not meant to be a sort of closed community only for marginalised people to get away from the corporate web, it's for everyone to use in whatever way they see fit.
There is literally nothing more to discuss if you're wondering why people "defend" Threads.
And that's fine, the Fediverse gives you tools to not have to deal with that through silencing or defederation.
But for many people on the Fediverse, they're here specifically for other things, and being able to interact with the corporate social web from outside of it is ideal for them.
Note that I'm not arguing for or against here, it's just very easy to see why many aren't interested in defederating.