That’s because they’re essentially defederating entities they don’t trust; exactly what’s being proposed here. The solution to defederation is not pre-emptive defederation.
They’re defederating smaller entities because the network got consumed by corpos. And abuse, but lots of that comes from big services and they don’t defed those.
Fediverse instances aren’t just providers, they’re communities.
That’s a real problem for the Fediverse too, because there’s almost nothing that stops someone from spinning up infinite numbers of instances and spamming other instances.
This is in essence what FB/Meta is doing, all the time, except it’s not individual spam it’s an algorithmically backed manipulation mechanism using it’s users as tools ^.^