Why Emacs has sound support? Well, you could ask that about many other things in Emacs, but sound support is relatively understandable since you want to be able to do things such as play notification sounds. That's for example how erc recommends the use of the sound system.
As to why ALSA and not PipeWire, well, for one, the support is from before pipewire was even a thing. The ALSA support even seems to be from the time when PulseAudio was new (2006 seems to be year of the first ALSA-related commits in src/sound.c.) And since it seems to work, I'm not sure that the developers are that keen to just going and replacing the ALSA support with PipeWire, especially given the latter's complexity.