Why not just use gcc which already exists everywhere?

When you answer that, same answer. If you can't imagine any answer for that, then the answer won't be convincing or make sense even if anyone tried to articulate it. Which is fine. Everyone doesn't have to find meaning in the same things.

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.