Pipewire is a weird case. It was made as a general multi media orchistrator, that includes video and audio. It takes a lot of stuff from different sources.

> Then why was PipeWire introduced as a PulseAudio replacement

One of them is putting more focus on permission management with support for sandboxed containers, which is kinda necessary with the direction the Linux desktop is going. But it also is just a backend, that supports jack, alsa and pulse interfaces, so it was mostly a drop in replacement for the applications.

Talk with the guy behind pipewire https://www.youtube.com/watch?v=_buKw1la_tg (4:34 for the motivation behind pipewire)

Pipewire was created to support containerized applications (with limited system access), to also support video, and to unify both pro low-latency audio (JACK) and regular low-power audio (PulseAudio). It might have been possible to retrofit this all into the PulseAudio project - but the authors decided to start from scratch. There are some interviews and talks on YouTube, they might have insights into why exactly that was the case. But I have not seen any indications that it was because PulseAudio did not work for what it was designed for.