The Fediverse is one thing that Big Social cannot acquire or copy.
No one owns the Fediverse, so that's not an option.
And to copy or integrate the Fediverse is to kill their own cash cows.
In essence, the Fediverse targets the chunk in their armour.
Big Social is obsessed with metrics, and proving value through metrics.
What metrics prove the Fediverse's value?
No one knows. Every metric regarding the Fediverse is at best conjecture. Nobody even knows its true size.
Hard to convince a CEO to buy into an idea when nothing is probable through metrics.