Why doesn't @rollup/plugin-typescript inline imported module contents when bundling multiple TypeScript files into a single JavaScript file?

[Autumn-one]

• Rollup Plugin Name: @rollup/plugin-typescript
• Rollup Plugin Version:12.1.1

Feature Use Case

I'm confused about the default behavior of @rollup/plugin-typescript. In my simple TypeScript project, I have just two files:

add.ts: exports an 'add' function
index.ts: imports and exports the 'add' function
When I bundle this project into index.js, I expected the content of add.ts to be included in the final bundle. However, I found that the bundle still contains the import statement instead of inlining the actual code from add.ts. This is not what I expected.

Feature Proposal

What I want is to bundle multiple TypeScript files into a single JavaScript file with all their contents combined.