The problem is that the entire point of instances is not just to spread the costs/load of hosting the network, but to allow people to find places where the culture and norms (and thus the moderation style and block lists) align with what they feel comfortable with, so they can rely on being able to find a place that will have a nice experience for them. For instance, I choose explicitly LGBTQ-friendly and anti authoritarian instances on the Fediverse. Automatically moving people around based on heuristics has the chance of really fucking that up for people. I don't want to wake up one morning and find myself on an instance that federates with whatever rightwing shithole instance will crop up in the next few months and have to avoid the platform or be harassed with dick pics and transphobia.

Additionally, having a centralized server for anything kind of defeats the point, no?