Why is serverless-bundle added as a dev-dependency in each service and not at package.json at root level?

[s1mrankaur]

Here: https://serverless-stack.com/chapters/share-code-between-services.html

It's suggested that: We use the package.json at the project root to install the dependencies that will be shared across all the services. For example, the serverless-bundle plugin that we are using to optimally package our Lambda functions is installed at the root level. It doesn’t make sense to install it in each and every service.

But in the project, you have added serverless-bundle as dev-dependency in each service. Why is that?