One main reason is performance. Forking for other tools is very expensive.

That said, using larger sed or awk programs instead of ad-hoc calls for small snippets would perhaps be net-positive for performance and readability.

I'm currently working on very strict bootstrap scenarios in which sed and awk might not be available, but a shell might be (if I'm able to write it). It is possible that in such scenarios, the fist send and awk versions will be shell-written polyfills anyway.