Point being, there's a lot of infrastructure that kinda doesn't make sense split up. If you want resiliency, build a second system in parallel not make management of an existing one times more expensive.
There is a little bit of wisdom out there: Don't put all your eggs in one basket.
Heterogenous architecture (where you intentionallly mix and match differing parts for increased fault tolerance) has been a thing for decades.
Defence in depth is expensive, infrastructure at scale is expensive, it would be difficult and more expensive to have a bunch of small operators reach the same level considering the initial cost and overhead.
I'm sure though that such equally viable alternatives could be built, but not by splitting existing ones.