It's right in their wiki, "non-goal" number 4 in their TS Design Goals which is also referenced again in some issues:
[a non-goal] Provide an end-to-end build pipeline. Instead, make the system extensible so that external tools can use the compiler for more complex build workflows.
[a non-goal] Provide an end-to-end build pipeline. Instead, make the system extensible so that external tools can use the compiler for more complex build workflows.
https://github.com/Microsoft/TypeScript/wiki/TypeScript-Desi...