Because bad monorepos ala monolithic apps tend toward no separation of concerns, and with a large enough team everyone is stepping on each other's toes in conflicts, tests, etc. Some tech like Rails makes this even harder to enforce boundaries.

I get that a good team can manage a monorepo that's not one big ball of code, and a bad one wouldn't necessarily be better with microsystems either.

But it's super frustrating to hear teams blobbing everything together with no layering or separation and defending it as "but we're a monorepo"