Service Oriented Architecture became a thing because monolithic code bases were often becoming spaghetti. I worked on a project where removing an option from a preference window (max map zoom), broke a message table (because the number of visible rows in a table (not its size in the UI) was linked to the max zoom you supplied to a map library, for no reason).

Thus the idea you should wrap everything you do as a self contained service, with a known interface. The idea being you could write an entirely new implementation of a service, implement the interface and everything would work. Microservices are a continuation of this idea.

Yet every node/python based mono repo I have seen will have python files directly imported filed from inside anouther component/service. Not simply common aretfacts but all sorts of random parts. Subverting the concept of micro service (and recreating the problem).

Separate repositories block this because each repository will be built in isolation on a CI, flagging the link. This forces you to release each repository and pull things in as a dependency. Which encourages you to design code to support that.

A common monorepo problem is to shove everything in a docker image and call it a day. Then if you need a class from one monorepo in anouther one, you don't have an artefacts so lazy devs just copy/paste files between monorepos.

Monorepos aren't bad practice by themselves, they encourage bad practice. Separate repositories encourage good practice (literally the need to manage them separately drives it).