well there is modularity and then there's keeping frequently used things together. I think react push modularity at the cost of productivity (opinion).

It's like breaking a kitchen knife into two: the blade and the handle, in the name of modularity.

Concretely, this is what I mean: Routing, and a bunch of other things are used together, it would've been better if react had kept the view library separate but also maintained an official router among other things

Routing isn't always necessary, if you're building server rendered pages and a non-SPA web app. Facebook isn't a SPA, so that's why they never built and subsequently released a router. They generally only open source and release things they use in production.