When you have shared resources between two services (such as React components), you have 2 choices: have a separate shared repo that will need to have its own versioning, and keeping it in sync with development is a pain if multiple people are working on features that touch both shared components and individual services at once, or have a monorepo where the service and shared components can just be worked on through the same repo.

The same story is true with things like APIs or types where two services need to stay in sync.